I guess it depends when you're going, but these are my suggestions:
Festen
Woman in White
Fully Committed
Mary Poppins
Jerry Springer: The Opera
Acorn Antiques
Grand Hotel (through Feb 12)
A Minute Too Late (through Feb 26)
On the Town (beginning March 5)
The Anniversary
The Far Pavilions (beginning March 24)
Billy Elliot (beginning March 24)
The History Boys
Whose Life is it Anyway? (starring Kim Cattrall)
Click the link for more info on these shows...
London Theatre Listing
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian