Let's see.
A stealth game with a ridiculous amount of secrets and play styles. No kill runs? No alerts? With an incredible amount of depth, variety, with fantastic boss fights (two hour long sniper fight in the jungle), fantastic enemy AI, multiple difficulties and unlockables. It is the best stealth game ever.
Versus Half Life 2.
An above average single player fps with average shooting mechanics, horrifically average AI, uneven pacing, and a gameplay style that doesn't really emphasize play styles but doing whatever Valve wants you to do at that point in time?
Metal Gear Solid 3 slaughters Half Life 2 in every regard: content, variety, pacing, boss fights, depth, difficulty, AI, memorable moments.