Mario and Sonic at the Olympics, definately mate. Wii Play is worth it too as for £40 you get a free controller and some decent games in there that get you used to the way they work and react.
If you want a couple of cheap games try rayman raving rabbids - really gets my arms aching - my 9 year old son beats me every time, or elebits (prob pick these up for about £15 each. My favourite has to Guitar Hero III, especially if you're a rock fan, but you have to buy the guitar and it all costs about £60
Deffo agree with Rayman Raving Rabbids - funny game - throwing Daisy the cow like a hammer throw in olympics really is funny.