Sri Ramakrishna was the greatest incarnation of God and spiritual figure seen by humanity till date. He had revealed His real nature to few of His disciples; He had said that 'one who was born as Rama and as Krishna, is now born as Ramakrishna'. Swami Vivekananda, Sri Ramakrishna's foremost disciple, had critically tested Sri Ramakrishna several times and through reason and experience, had come to conclusion that no other saint or incarnation in recorded history of the world can match the spiritual measure of Sri Ramakrishna. The sheer scale and volume of Sri Ramakrishna's spiritual practices is itself unparallel, as to the spiritual power He wielded, several instances can be found in reminiscences of His disciples.

When Sri Ramakrishna was on His deathbed, filled with bliss within but bodily suffering intensely from throat-cancer, in Swami Vivekananda's mind there arose a doubt regarding Sri Ramakrishna being Incarnation. 'Life of Swami Vivekananda by His Eastern and Western Disciples (Vol I)' narrate the incident as below:

'A couple of days before the Master's passing away, when Narendra and a few others were standing by his bed at night, a curious thought flashed across Naren's mind: "The Master has said many a time that he is an Incarnation of God. If he now says in the midst of the throes of death, in this terrible moment of human anguish and physical pain, 'I am God Incarnate', then I will believe."
No sooner had Naren thought this than the Master turned towards him and, summoning all his energy, said, "O my Naren, are you not yet convinced? He who was Rama, He who was Krishna, He Himself is now Ramakrishna in this body: not in your Vedantic sense [according to which each soul is potentially divine], but actually so."
Naren was dumbfounded. He was stricken with remorse and shame for having doubted his beloved teacher even for a moment, in spite of so many revelations he had had from him (the Master) in the past. By these words Shri Ramakrishna conveyed to his disciples the momentous truth that the personalities of all the divine Incarnations of the past were the same Personality (the Purushottama), revealing Itself variously in different human forms according to the needs of the time: nay more, that he himself (Shri Ramakrishna) was that Personality--which is called God.'

During His spiritual practices, Sri Ramakrishna had followed various spiritual paths enjoined in Hinduism like Vaishnava, Tantra, Vedanta etc. Later He also followed Islamic and Christian modes of sadhana. Through all these paths He reached the same Reality of Infinite Pure-Consciousness-Bliss. We find no other example in spiritual history of the world about someone who had actually practiced and verified the validity of so many paths and experientially proven that all paths and faiths lead to the same goal. This has heralded, in this modern age, new hope for inter-faith reconciliation and removal of narrow-mindedness from religions.

As to remaining in the highest state of spiritual illumination - Samadhi - for longest duration, Sri Ramakrishna had broken all records and in fact has set such a high record that it seems almost impossible that anyone can match it! He remained in Nirvikalpa Samadhi (highest spiritual absorption beyond mind) for continuously six months! His body was saved for humanity during this period by a monk who arrived there by Divine Will.
And as to manifesting high spiritual moods, which take it's toll on body, He Himself says, "The manifestation in the same individual of the nineteen different kinds of emotion for God is called, in the books on bhakti, Mahabhava. An ordinary man takes a whole lifetime to express even a single one of these. But in this body [meaning himself] there has been a complete manifestation of all nineteen."

Swami Saradananda, a direct monastic disciple of Sri Ramakrishna and God-realised soul, has written his magnum-opus 'Sri Ramakishna LeelaPrasanga' which, not only accurately details Sri Ramakrishna's life, but also provide the philosophical details, which are required for writing an authentic biography of an Incarnation of the magnitude of Sri Ramakrishna. 'Sri Ramakrishna and His Divine Play' by Swami Chetanananda is an able modern English translation of this Bengali Classic. In the Introduction section Swami Chetananandaji observes: 'This book bears witness to Sri Ramakrishna's testimony that all religions are equally valid. He found a place for each one in his own life. He first realized God by following Hindu practices, and then by following the Christian and Muslim paths. Such a journey is unique in the religious history of the world. He afterwards proclaimed, "As many faiths, so many paths," thus establishing an ideal harmony of religions for our present age, in which religions are in conflict and hatred and violence are rampant. He taught a religion so badly needed today, a religion that is constructive and not destructive, scientific and not fanatical, practical and not theoretical, rational and not superstitious, universal and not parochial. Truly, Sri Ramakrishna worked to create unity in our time and he repeatedly stated that the goal of human life is to realize God.'

One can find many instances in this book, which prove that Sri Ramarkishna was indeed greatest Incarnation of God.
Some of the important conclusions Sri Ramakrishna Himself deduced from His life about Himself and spirituality in general are neatly put in the 'Gospel of Sri Ramakrishna (Introduction Section)':

'First, he was an Incarnation of God, a specially commissioned person, whose spiritual experiences were for the benefit of humanity. Whereas it takes an ordinary man a whole life's struggle to realize one or two phases of God, he had in a few years realized God in all His phases.
Second, he knew that he had always been a free soul, that the various disciplines through which he had passed were really not necessary for his own liberation but were solely for the benefit of others. Thus the terms liberation and bondage were not applicable to him. As long as there are beings who consider themselves bound, God must come down to earth as an Incarnation to free them from bondage, just as a magistrate must visit any part of his district in which there is trouble.
Third, he came to foresee the time of his death. His words with respect to this matter were literally fulfilled.

About spirituality in general the following were his conclusions :
First, he was firmly convinced that all religions are true, that every doctrinal system represents a path to God. He had followed all the main paths and all had led him to the same goal. He was the first religious prophet recorded in history to preach the harmony of religions.
Second, the three great systems of thought known as Dualism, Qualified Non-dualism, and Absolute Non-dualism - Dvaita, Visishtadvaita, and Advaita - he perceived to represent three stages in man's progress toward the Ultimate Reality. They were not contradictory but complementary and suited to different temperaments. For the ordinary man with strong attachment to the senses, a dualistic form of religion, prescribing a certain amount of material support, such as music and other symbols, is useful. A man of God-realization transcends the idea of worldly duties, but the ordinary mortal must perform his duties, striving to be unattached and to surrender the results to God. The mind can comprehend and describe the range of thought and experience up to the Visistadvaita, and no further. The Advaita, the last word in spiritual experience, is something to be felt in Samadhi, for it transcends mind and speech. From the highest standpoint, the Absolute and Its manifestation are equally real - the Lord's Name, His Abode, and the Lord Himself are of the same spiritual Essence. Everything is Spirit, the difference being only in form.
Third, Sri Ramakrishna realized the wish of the Divine Mother that through him She should found a new Order, consisting of those who would uphold the universal doctrines illustrated in his life.
Fourth, his spiritual insight told him that those who were having their last birth on the mortal plane of existence and those who had sincerely called on the Lord even once in their lives must come to him.

Once Swami Vivekananda was asked by one of his disciples: 'Do you, may I ask, believe him to be an Avatara (Incarnation of God)?'
Swamiji replied, 'Tell me first -- what do you mean by an Avatara?'
Disciple: Why, I mean one like Shri Ramachandra, Shri Krishna, Shri Gauranga, Buddha, Jesus, and others.
Swamiji: I know Bhagavan Shri Ramakrishna to be even greater than those you have just named. What to speak of believing, which is a petty thing -- i know!

And once Swamiji said to the same disciple: 'You see, the fact is that Shri Ramakrishna is not exactly what the ordinary followers have comprehended him to be. He had infinite moods and phases. Even if you might form an idea of the limits of Brahmajnana, the knowledge of the Absolute, you could not have any idea of the unfathomable depths of his mind! Thousands of Vivekanandas may spring forth through one gracious glance of his eyes!'

The following write-up by Swami Vivekananda is also greatly illuminating on this topic:
Hinduism and Sri Ramakrishna

Let us make our lives blessed by studying some of the quotes of Bhagawan Sri Ramakrishna. May He bless us all!