Ahhh come on, 5 isn't enough.
I've had to say...
1. Final Fantasy VII
2. Medal of Honor: Allied Assault
3. Age of Empires II: The Conquerors
4. Call of Duty 4: Modern Warfare
5. Half Life
Others include,
FFVIII, FFIX, Mass Effect, both Knights of the Old Republic, Metal Gear Solid, Chrono Trigger, Sensible Soccer, Pro Evolution Soccer series, CM 03/04, EVE Online, I could just keep going really.
One game that will forever stand out in memory is the first Silent Hill on the PSX. I had it imported and didn't have a PAL Converter so it was in black and white which just made the ****** more scary when I was like 14. Also, anyone remember Chase the Express? I thought that was quality back in the day, just looks wank now.