I assume we're talking about the BBC version; my favorite in that case would be Peter. I had a huge crush on him growing up, and still prefer his performance to Mosley's. I think that every other Pevensie in the BBC was not so great and even terrible (Lucy). Caspian, Eustace and Jill were all good in their ways, but nobody stood out the way that Peter did.
Funny thing, I watched the LWW again after many years and discovered that Peter was shorter than Edmund...
That was a shock. I never noticed that, and even then I thought the big-brother bit was pulled off extremely well.
[Edit] I haven't seen the BBC versions for a long while, I saw the BBC LWW most recently about 5 years ago, so even that's been a while. I don't really remember any of the characters in them (except SC) so my opinions may be a little vague. Except Peter... he's still cute.