Thunder Bs are always awesome. Everyone loves em, and if you mod them a tiny bit you make them way more reliable and fun. None of that 10 second fuse crap.
And as you can see, filling with BBs has a great effect. It also adds some weight/range to the grenades as non-BB ones are harder to aim or get great range on. Good range, yes, great range, not so much.
Another type I've always been interested in is the TLSFx grenades. Yes they do use pyro, but I would still love to get some.