Super Street Fighter IV.
This game is totally awesome. If you own an Xbox or a PS3, or PC or whatever, you should get this. You'll know why you did.
I was a huge fan of Street Fighter before 4, having owned the Anthology on PS2 and Street Fighter 2 on the XBLA. But Street Fighter 4 was a massive step up. It made me want to get serious about Street Fighter because combos just seemed so much more accessible thanks to the Trial system for each character. And also being able to play online (which worked very well), was a good way to see how others play the game and what combos others have learned.
But now, Super Street Fighter IV has stepped up AGAIN where I thought it not possible. 10 new characters, possibly 12, a new ultra combo for each character, new online system allowing more than 2 people to play at once, more balanced combat (Gief's ultra was way over-powered in the original), more combo opportunities, new stages, better (less cheesy) music, new menus with new game modes, and I'm sure there's more. And as you play you get better and your online grade improves, as you continue to learn and win more. I've had Street Fighter since when it came out, and Super Street Fighter since it came out, and I'm still learning and finding out new things with characters I never thought I could win with.
You definately need to at least try this game. At £25 maximum, this is just an awesome game at an awesome price.