No order,except the first two.
-HeXen : I've spent whole DAYS to finish it,the puzzles are kinda challenging(the items are hard to find

) but it was worth it.Perfect enemies/level design.
-Sacrifice : Great story,great characters,great story,giant replay value,awesome creature design.All in one.
-Heretic : See above,only that I spent less time to finish it.Basically it's HeXen with no puzzles and a lot of monsters.I love the
Iron Liches' design.
-Cave Story/Doukutsu Monogatari : This one has awesome replay value.
-Ghost Master : Finished it twice,now started to switch .bsp s and get more GP.Darkling/Buck switch is...
-Legacy of Kain/Soul Reaver series
-Laxius Power/Force Trilogies
-Beneath a Steel Sky
-Rome:Total War
-Age of Empires series
-Sam and Max:Hit the Road
-Sam and Max:Season 1/2
There are a few others,but my lazyness stops me from typing any more.