As previously stated, Gran Turismo 5: Prologue, it isn't the biggest game in the world but it can be had for £15 which is well worth the money.
Uncharted, obviously
Super Stardust: HD is just awesome for the money, i've put off buying it for ages but now i own it i wouldn't ever want to lose it
Warhawk can be fun, i'd say it's worth it for £20 off the PSN
While everyone recommends Tekken, i wouldn't say it's worth the £15, all the characters bar 1 are pre-unlocked, giving single player very little value and ive never been able to get a lag-free game online.
GTA4 is great, but gets a little samey
Heavenly sword is good but short, either rent it or pick it up second hand for £10/15
The Darkness is fantastic for the £18 it costs these days.