I am going to list here, with minimal commentary, the lineage of teachers which constitute what we refer to as Tibetan Buddhism.
Firstly, Tibetan Buddhism is essentially North Indian Buddhism - a style of Buddhism that largely disappeared everywhere else in the world. But that is not entirely true - actually it has received Buddhist influence from all kinds of Buddhism, specifically, Kashmir, Afghanistan, China, and the pre-Buddhist Tibetan magical and animist beliefs and practices.
Historically, the first introduction of Buddhism to Tibet occurred in the age of the Kings. We can call this the 'second' period, as prior to this there was little cohesion, but a culture nonetheless. During the age of the Kings, Tibet became a world power, invading China and even conquering its then capital, and across to the west where it came up against the Islamic/Mongol empires.
Early Buddhist influence came through one of the famous King's Chinese wives (the Jo-khang temple and image date from this time). Then there was the Great Debate, about which I have already written. At this time Buddhism also came in from India.
The notable ancient primary influences:
Santarakshita, the 'Bodhisattva Abbot', founder of the first monastery Sam-Ye (bSam-yas).
This man never seem to be fabulised. He was the first real Buddhist teacher, and a very sober, true student of Buddhism. I haven't read much about him personally or his specific teachings, but he is no doubt the true originator of Buddhism in Tibet.
Padmasambhava, 'Guru Rin-po-che' (Precious Master). Yogin-sage, skilled in magic and mysticism, most probably from Swat in what is now Pakistan.
Commonly thought to have been invited to Tibet by Santarakshita, as his magical skills were more in tune with popular culture. There is a lot written about Padmasambhava, and he is definitely the most loved of all Tibetan gurus. Likely most of what is written has been fabulised and glorified.
I should caution Western-trained minds against the restriction of 'truth' to realism. Myth hold much knowledge, and it is a mistake to reject fantasied accounts as erroneous.
Guru Shenrab (gShen-rab) - the legendary Bon originator.
Quick overview: pre and early Kings period, religious practice was called Bon ('Invoker') and Shen (gShen) ('Sacrificer'). During the end and after the time of the Kings, there was persecution of both Bon and Buddhism, during which period - the third period - both religions reorganised themselves. Buddhism essentially through influence from North Indian Buddhism, and Bon through the same source, but more from the non-monastic individuated practices of Buddhism. In essence Bon replicated everything in Buddhism, with minor twists.
Shenrab was probably a real person, but his place in Bon is largely a regurgitated version of Sakyamuni, who had plenty of mythical content in his story also.
The forth period. This is when Buddhism re-established itself in Tibet. The personages mentioned here are but a small set of what was thousands of enlightened teachers over many hundreds of years. The only reason we know of these is because of wealthy patronage. And with these famous teachers, I can only give a very small selection - they all had disciples of disciples of disciples who are very famous in Tibetan Buddhist history, and well known even today. The lineage cris-crosses with India, so it is there we begin.
Each of these is well referenced, and you can find much material on all of them,
Tilopa, the 'Great Magician'. He was in India, and much of what became Tibetan Buddhism stems from this man.
Naropa, also the 'Great Magician' in India, disciple of Tilopa. The stories of what Tilopa put Naropa through are legendary. Both these men formed the base of Tibetan mystical traditions.
Santipa, another 'Great Magician' in India.
Brogmi (992-1072). Traveled to India and studied under Santipa. He is know as the translator and commentator of the Hevajra-Tantra text. He founded the Sa-Skya Order, one of the four main Orders of Tibetan Buddhism. But not quite, actually it was his disciple Konchog Gyalpo of the wealthy and influential Khon family who spotted an auspicious site for the Sa-skya monastery, which turned out to be auspicious indeed - achieved vast wealth and power. Konchog Gyalpo was an eccentric mystical wonder-worker, much admired in Tibet.
Marpa (1012-96), the Translator, disciple of Tilopa. Founder of the Kagyud Order.
He first studied under Brogmi, but then went himself to India and learnt from Naropa. There are many great stories of Marpa, and he is probably the most well known of the Order founders. The story I like personally, is how he lived the outward life of a husband/farmer, and only very few knew he was one of the foremost yogins of Tibet.
Milarepa (Mi-la Ras-pa), disciple of Marpa, the most famous and popular of Tibetan yogins, renowned for his mystical powers and songs. He needs no introduction. He was an ascetic and discourage followers. But one man persisted - Gampopa (sGam-po-pa), who really established the Kagyud school.
Gampopa, whose direct disciples established six famous schools, all based on his teachings. He is truly one to read up on.
Phagmo (Phag-mo-gru), one of Gampopa's greatest disciples, established the first Kagyud monastery, Densatil (gDan-sa-mthil). Phagmo lived in a small grass hut, around which dwellings of his followers were built until after his death it grew into a great and wealthy monastery, still surrounding and revering his miserable grass hut.
Atisa. He was a highly revered and famous scholar and teacher/magician in India before he was invited by the Kings of Gu-ge to come to Tibet in 1042. This man had taught at three of the great Buddhist universities in India, Bodgaya, Odantapuri and Vikramashila. When he arrived in Tibet (which he never left), he transformed Tibetan Buddhism from then on. His prestige and personal power was so great, that he set the seal of Buddhism in Tibet in a way no other teacher had the authority to do. He is definitely one to read up on and study. He brought a revival of the monastic and disciplined approach to the religion.
One interesting thing is that when Atisa translated the Guhyasamaja-Tantra ('Tantra of Secret Unity') he replaced the central set of five Buddhas in this text with Lokesvara ('Lord of the World' - Shiva), substituting an accepted Buddhist symbolic arrangement with a devotional divinity. From which time this change characterised all Tibetan Buddhist practice. However this was really only an extension of changes current in Bihar at that time, the last centuries of Indian Buddhism.
This change is curious and seems to represent a repetition of the earlier permeation of Shiva up through the overlay of Aryan Vedic religion. The point is that despite the strength of priest/monk allegiance to pure Buddhism, the population were happy to revere sincere spiritual people of all persuasions but never found it necessary to give up their traditional worship - they just added new ones in. Thus over time Shiva began to rise into Buddhism by osmosis, until you had Mahakala, worshiped by Buddhists and Hindus alike. Then Islam wiped out Buddhism across India, but never could defeat Shiva.
The forth period - the modern period - I will deal with subsequently.
