Best Offensive: It's kind of hard to say, there are a lot of excellent offensive sweepers. It doesn't just depend on the typing either, some pokes with great typing have a really ordinary ability to go with it making it not as good as it should be. I've found Arcanine to be a really solid attacker, it's pure fire type yes, but with its GEN 5 movepool and maybe an air balloon, there's really something to work with.
Best Defensive: Like I said above, alot can depend on ability as well.
For instance, it's not easy to pull off but you can manage a skill swap chain shifting wonder guard from Shedinja through another poke and then swapping it finally into Sableye or Spiritomb. Ghost/Dark is probably the best defensive typing in my opinion, having no actual super effective weakness.