Traditionally, Realisation is seen as a supreme state that is very difficult to attain, requiring a lifetime of rigorous practice, discipline, and aspiration. We here of tales such as the story of Gautama Buddha who renounced life as a prince in an idylic (and fantasy; the whole story is as heavily mythologised as any fairy tale) world free of suffering and illness (didnt he ever even catch a cold?) and left his wife and young child to live the life of a renunciate and spend many years of spiritual striving and physical austerities in which had prepared him psychologically and spiritually, finally sitting for three days in constant meditation under the Bo Tree (Awakening Tree, the sacred Fig Tree), after which he was totally Awakened and liberated.
Later mythology obscured Siddharta Gautama's human atatinments by placing him in the realm of the Divine. According to Mahayana Buddhism, the Buddha was born Awakened, and his quest for transcendent Awakening ("Enlightenment") (bodhi or sambodhi) was only a "skillful means" to guide and instruct others, however this may simply be a later mythologisation and apotheosis, just as some Sufis identified Mohammad with the cosmic logos (see some of the books of Henry Corbin for example). Interesting as this may be from a theological and transpersonal psychological (in terms of Jung's archetype of the Self) perspective, it does not throw more light on the subject of realisation; indeed it only obscures the subject. I have also noticed a similar process of mythologization around Sri Aurobindo, regarding opposition to a recent non-hagiographic biography.
Interestingly, Sri Aurobindo also spent three days in constant meditation (using a Samkhyan-type technique taught to him by a yogi called Lele, about whom little is known), after which he had attained the state of Brahman, the realisation of the silent Self.
For Ramana Maharshi, the experience was relatively mild, it constituted a simulated near-death experience. This would indeed indicate that he was already Realised; there was only the slightest veil obscuring his divine nature. Meher Baba had a number of meetings with spiritual sages, who helped him to realise his divinity. However, Meher Baba's extreme claims to be the one and only deity make me wonder if he wasn't actually more equivalent to Da Free John (High Intermediate Zone), and perhaps also Sathya Sai Baba, although without the abusive behaviour associated with the latter two.
In any case, the concept of the Intermediate Zone indicates that realisation and nonduality of a more partial sort may actually be very common, whereas it is the burning away or transmuting all the lower desires, samsaras, and kleshas that is difficult. This is not to deny that intermediate zone experiences may not be very profound or elevated; often they are. I have tentatively suggested a number of categories here.
Realisation thus seems to involve a number of stages by which the samskaras and kleshas are removed. and if the person is already very highly spiritually evolved, this sort of experience may not be rare, and may involve only the most trivial-seeming contact with a Sage. Arunachala Ramana achieved (intermediate zone?) realisation just through looking at a picture of Ramana Maharshi, although he didn't even know Ramana at the time.
For a while, I suggested the term Transenlightenment for a state even beyond conventional "Enlightenment" (Realisation). As I no longer use the word "Enlightenment" there is no point saying "Transenlightenment" either, although Sri Auriobindo's description of states beyond conventional Realisation and Liberation, such as Overmentalisation and Supramentalisation, does certainly seem to constitute a higher grade of being. Hence the distinction between Realisation that confers individual Liberation, and a collective state of Divinization which leads to the Awakening or Perfection even of non-conscious matter itself (Supramentalisation/tikkun olam - cosmic restitution)
There is also a question whether or not an Authentically Awakened or transcendentally Realised teacher is born Realised, but has to partake of a certain veil of limited humanity, or starts out imperfect and has to attain Perfection. Perhaps either might be the case. Regardless, the Realised teacher might attain the Supreme through various means, such as different successive Masters (e.g. Yogi Ramsuratkumar), intense meditation (e.g. Guatama Buddha, Sri Aurobindo) or through an ego-death experience (e.g. Ramana Maharshi), or even being struck on the head by a sage (Meher Baba, assuming the latter is authentically Realized). After that point, the teacher is fully self-Realised (or partially self-realised but still with some sense of inflation, if an Intermediate Zone guru), their identity one with the All.
This list only includes those of the 19th century through to the present day. Doubtless many more historical examples could be added. The problem is, the further back in history one goes, the more things become veiled in myth and legend and exageration (indeed this is so even with the modern day as the phenomenon of urban folklore shows). There was a time when I would dogmatically assert taht this or that guru was authentically Enlightened (I even thought some that turned out quite imperfect were Enlightened. Now I am much more cautious, as well as acknowledging that the division between Intermediate Zone and Realization is not so clear cut. Consider for example Meher Baba and Da Free John each extremely powerful, charismatic, and profound individuals, yet each also claiming to be the one and only Godhead. Whereas some are extremely humble, such as Nisagardatta Maharaj and Yogi Ramsuratkumar, but if they are fully Realized they don't have the power of either "Intermediate Zone" avatars like Meher Baba, Sai Baba, or Da Free John, or "nonduality avatars" such as Ramana Maharshi, Anandamayi Ma, or Nityananda.
In no particular order then except alphabetical:
(note: Vivekananda is traditionally included here, however I was surprised to discover that neither Vivekananda nor Ramakrishna were vegetarian, which indicates a lack of empathy regarding non-human life. On the other hand, there is a reference on Wikipedia which states that Vivekananda was vegetarian. Perhaps like Sri Aurobindo he became vegetarian later in life. Certainly he spoke out strongly against animnal scarifice. Also, as Ken Wilber I believe correctly points out, development does not have to proceed uniformly at all levels. Even Sri Aurobindo ate meat, smoked cigars, and drank alcohol in the early days).
Because my interest is in Divinization, I tend to emphasise Realized beings of that exceptional degree. There would however be a huge number of Realizers who are "only" at the stage of authentic Realisation
Some historical figures
many many more could certainly be added....