I don't think either of these levels are the best for any gametype but I think they are good for evey gametype.
Beaver creek is a simple, nearly symetrical map good for slayer assualt and multiflag. Most levels in halo are only good for one specific gametype and I love that this map can do it all.
I like Zanzibar for a completley different reason. Its an asymetrical map perfect for slayer one flag and one bomb gametypes. I think its unique in it being the first level built with those two one sided gametypes. And I could be wrong but i thought it was also the first level to have a moving piece with having the wheel in the middle.
I'd have to say Headlong because it always had me feeling uneasy and on my feet. Enemies can be hiding anywhere or pop out from like 12 different places and just take you out. That was the ultimate camping map, if you had a team that played the stealth role you were in for some serious trench warfare type paranoia..
woa! not sure if you're joking, but that just may be one of, if not the worst map that's ever been made for Halo. The map is so bad in fact, that it was the only map that was removed from Halo 2's matchmaking, haha! If you do like it for some reason, that's fine, but it was very bad haha.