By far my favourite Alien/Predator game out there is AVP2 for PC. No other AVP game around really gives u that great special feeling that u have when u watch the classic movies. It's like playing through both the Aliens, & original Predator movie, the best of both worlds. Multiplayer is great (albiet the slightly clunky melee combat system) overall, easily the best out of all Alien/ Predator games in my opinion.
Concrete Jungle is my second favourite on the list. While the AI was terrible & I would've preferred if the preds' theme from the game wasn't so strongly influenced by AVP, it was still a reasonable enough game, although it could have been a bit more challenging, as u don't really feel like ur hunting intelligent prey that actually poses a serious threat, as the AI is so terrible u feel like ur hunting a bullseye with legs.
Oh, & on a side note, AVP: Extinction isn't an RPG, it's an RTS (real time strategy)