Metal Gear Solid 2: Sons of Liberty. I've been playing the MGS games in order, and I love them. I got Metal Gear Solid for $9.99 and it was badass in every way possible (Psycho Mantis has to be one of my favorite boss fights ever), and I bought Sons of Liberty not too long ago for $6.99. It's definitely worth it, despite how much of a pussy Raiden is. I just beat the fatman boss, which was also badass.
I'll borrow Snake Eater from my friend, who has all of them but hasn't finished the first one yet. I have no idea what to do about Guns of the Patriots, as I don't have a PS3.