In my humblest of opinions:
(1). Jedi Knight/Mysteries of Sith.
The successors to the amazing Dark Forces, i believe that this was the first "True" Star Wars game that delivered very addictive gameplay, excellent multiplayer (still being played even today) and was the first SW game to employ a proper 3D lightsaber combating system. When i'm bored, i still play my good old JK CDs and the game, like DF can still keep me hooked for hours (especially online).
(2). Final Fantasy VII
What can i say? One of the greatest SP RPGs of all time. This is my definate fav. out of the FF series. The first 3D FF game the game had an epic story of good vs. evil, had a beautiful battle design and had arguebly the most iconic villain in gaming history (that's right, the good 'old Sephiroth). The storyline within the game is kinda relevant nowadays, what with all the pollution and Global Warming talks! In short, Mr. Hironobu Sakaguchi should be proud for such a classic masterpiece!
(3). Medal of Honor: Allied Assault.
The first MoH to hit the PC, i believe it is arguably the best in the entire franchise. Combined with the equally as epic and well designed "Spearhead" and "Breakthorugh" the game consists of over 50 missions to trawl through which provided i believe to be the best WW2 gaming experience to date. The multiplayer online is insane as i'm sure people who've played it are aware. I'd recommend this over the CoD series as MoH seems to get less repetitive and can keep you hooked for hours upon hours.
(4). Command & Conquer: Tiberian Sun
The sequel to the amazing Command & Conquer: Tiberian Dawn this game is definately my favourite RTS game, and is the RTS game that got me into the genre. Both mission discs, the Global Defence Initiative and the Brotherhood of Nod provide two very different story perspectives and with the very well scripted cutscenes the game immerses you into the warring world and story. With plenty of gaming options, playable characters and vehicles to choose at your disposal the game still provides a level of complexity that a lot of modern RTS games fail to realise. The expansion pack "Firestorm" adds to this complexity even further with more missions and even more player options and more advanced an realistic AI to make it in my opinion one of the most complex RTS games to date. I vote it as the best C&C game and to people who haven't played it, all i can say is GET THE GAME NOW!!
(5). Half Life
To put it simply, one of the best - if not the best FPS' to date, it's also the best selling with approx. 8,000,000 copies sold worldwide. A very immersive storyline creates a realistic environment, and the gaming physics (at the time) were complex and amazing considering there were level interactions such as puzzles etc. and so the game was not just your average shooter. The expansions "Opposing Force" and "Blue Shift" just add to the greatness of this classic and add to boot the vast TCs that are out there (such as the very entertaining They Hunger and THE best mod ever made for a game, Counter Strike) the game now has a vast amount of options to keep you hooked for life - seriously the game is like a drug!!
Well, that's my fav. 5 PC games in a nutshell!