Battlefield 2 seems to be the logical choice, since many of us (myself included) play it. It's online only, however, unless you like playing with stupid bots. I'd much rather have a great SP game with a MP element tacked on, than the other way around. Thankfully, BF2 has a huge community and is actually a great game itself. Another perk is that BF2 is old enough to be dirt cheap, and you can pick it up along with its expansions for around 30 USD.