First of all, the most groundbeaking games were, without a single doubt, the following:
6) Lode Runner
Groundbreaking games for me (and just me) were the following:
1) Commander Keen E4: Goodbye Galaxy
(Commander Keen was the first side-scrolling platform on PC and a really famous platfromer of the good, old, DOS times. However, it was Episode 4 that was by far the best of the series, if you ask me, not that the episodes 5 and 6 were worse, but they lacked the variety of the 4th episode in both enemies and environment.)
(Truly amazing lode runner clone, but with much-much more features than the original, like a jetpack with witch you could fly, and a tremendous easy-to-use level editor!)
3) Wolfenstein 3D
(First pure FRS ever!)
4) Commandos: Behind Enemy Lines
(The king of stealth games, if you ask me!)
5) Serious Sam 2
(I love its arcade FPS style combined with cartoonish graphics and a good sense of humour!)
(It was the first RTS which had a serious tactic system, an excellent simulator of the Roman legion!)
(Lovely platformer with 3D graphics, but 2D gameplay)
8) Knigts & Merchants
(Very well-made strategy game, where resources gathering was as important as the combat.)
9) Total Annihilation: Kingdoms !!!
(Best RTS ever, period! It belongs to the few 14 years old games which has stil an active community! http://kingdoms.heavenforum.org/)
10) Worms 2, Armageddon, 3D, 4 Mayhem etc.
Well, that's all folks!