I've got a mix of PC and PS2 titles and they are as follows:
1. Battlezone - Easily the best mix of strategy and first person simulation. This game is almost 10 years old and still nothing has come up to match it. Build your offensive and defensive resources, manage raw materials and participate in missions. You could command teams ala Command & Conquer and also lead missions and snipe enemy pilots ala Battlefield 1942. And this was in 1998 !!
I keep wishing someone would pick up this game from Activision and create a worthy sequel. Battlezone 2 was a complete let down and was obviously rushed to market without a good understanding of what made Battlezone so popular.

2. Quake 3 Arena - First game to really get the arena style, never-stop-running, jump-twist-shoot style of game play so common now. If this game didn't invent the style, it sure did perfect it. When all you want to do is blow somethin' up, this is your game.

3. Soul Calibur - This game seemed to have the best balance of features, moves, atmosphere and playability. Whether you played as Cervantes or Kilick or Soung Mina the game gave you new reasons to keep comming back.

4. Return to Castle Wolfenstien - This was a fantastic sequel to the first-original first person 3D shooter. The sequel had a great ambiance and very good game play. The levels weren't too hard or too easy. The plot was not too involved, but deep enough to keep you interested. Good mix of weapons and lots of fun moments.

5. Half Life 2 - Since this game came out I've played it many times. Even after Far Cry and Quake 4 and Doom 3 and Area 51, this game still stands alone. The plot and graphics and weapons and scenery are all amazing. The variety of required skills and techniques to advance keep the game challenging. I'll be enjoying this game for years to come.