Wiley's my all time favourite grime MC. He invented the genre and is, in my opinion of course, the best to do it. Obviously you may scratch your head at his mainstream releases like Heatwave, but I think it's just testament to how well he understands music and how to make a living from it. His 'It's All Fun and Games 'Til' mixtape last year was fucking brilliant. Here's one of my fave freestyles by him:
http://www.youtube.com/watch?v=-3dJ-_qE0N4
Edit: clearly the simple video embedding instructions are lost on me