The exact definition of high level depends on how old the server is. Players of level 30, for instance, can be considered "high levels" in a newly created server. However, world transfers mean even new worlds can have level 100+ characters. Long running servers have more experienced "high levels" where level 100 is a kind of reference mark. 3-digits makes it a good definition as an obvious mile stone. Additionally, 100 is above the level 80 mark, which is important because it allows all vocations access to great potions.
Bubble is still the most famous since she held the "highest level trophy" for a very long time. Cachero, Eternal Oblivion, Mateusz Dragon Wielki and Seromontis are also good examples of "high level" players. Achieving a high level on your own is a remarkable fact but nowadays it is losing part of it's prestige due to lots of account sharers and botters.