Bug type moves have to be my favourite. Hah da da da da, just walking along, laughing at all the weak Bug pokemon with their weak moves, and then BAM. You been Megahorned. There are a load of different Bug type moves that just come out of nowhere and destroy.

Also, my Rapidash in Platinum had Meghorn, and it was obscenely useful. I base my party around what moves can be learned, and I always make sure to have at least two different Pokemon with Bug moves.

In terms of Pokemon type, as opposed to move, Water is a sound choice, and difficult to beat. But Fire will always have a special place in my heart. Most Fire types look really cool or cute or badass, except Ninetales which is all of them. Sunny Day is shockingly useful, as not only does it power up fire types, it also reduces Solarbeam to a one-shot move. If I have a Special Attack Fire Pokemon in my team, there's a good chance they'll be built around Sunny Day. It has another benefit, I swear, but I can't remember it off the top of my head...