Oldie but a Goodie!
Still holding up to today's FPS gameplay standards (Ravenshield was made in 2003) - it's great single player, but an absolute blast as a MP.
For those who don't know much about the R6 series, it's based of the novels by Tom Clancy. You control a team of elite tactical response officers (Swat, basically) - and go in rescuing hostages, killing terrorists, and everthying in between.
The series has always been hard, and keep in mind this is a realistic tactical FPS (no unreal tournament here!) - so, 1 shot kills are typically the way to go. This is where MP comes in, as a team of human players is fantastic fun, covering each other, throwing grenades and flashbangs, and going in with silenced automatics the nature of the beast.
The AI is pretty intuative (especially back in 03) - although you do have moments that glitch and you can exploit. This game is still supported by plenty of servers around the world so you can typically find a lobby and jump into the action.
Great fun, vintage game, and still worth a look.
GOOD:
Great FPS Action. True, tactical play. MP is fantastic. Vintage game that is on the cheap, great choice of missions and weapons
BAD:
Slightly dated. AI can be glitchy, difficultly can be unforgiving.