A huge fan but never heard of Dawn of War? U should rephrase. Dawn of War is so popular it even got to the World Cyber Games.

As for the main question, I'd say the answer is so obvious; Starcraft

Diablo 2 ?

Warcraft 3 ?

Baldur's Gate 2 ?

Starcraft Brood War ?

Red Alert 2 ?

Counter Strike 1.6 ?

Classics like those never die i guess

For Unlimited Gametime Experience - UT3, coz the UT series had countless gamemodes + mods + maps + new experience each game. You can even have fun without ever going online at all, courtesy of UT's unsurpassed competitive A.I Bot

For Competitive Gameplay - UT3, coz thats what its widely known as, a competitive game

For Solid Single-Player Gameplay - Crysis, coz the game emphasizes on Single-Player, well at least thats what the previews said

For User-Editability - UT3, proven by its huge mod community for the previous game (although Crysis also promised a powerful editor, shown by its trailers)

For Visual Beauty - Crysis of course. UT3 has nice graphics quality, but Crysis is potentially the most beautiful game ever I guess.

Well, I think those are the most important factors in an action game. 3-2 for UT3, so I'd go for UT3