I'm just basing this on my favourite types to use, not in order of 'best' or 'strongest'. I'll do my top 5
1. Fire. I've always loved the Fire types, ever since I picked Torchic as my first Pokemon! I prefer double-typed Fire Pokemon than pure Fire. I've always liked the Fire/Fighting Pokemon but I hope that they'll change it up a bit in Gen 6.
2. Ice. My favourite Ice type is Mamoswine, which is Ice/Ground. Ice is great for taking out the Dragons, but obviously has major weaknesses, although I've always liked them. Except Jinx. Eww.
3. Electric. Having only one weakness, Ground, Electric seems like a pretty formidable type to me. My favourite pure Electric types are Luxray, Ampharos and Blitzle, although Zebstrika doesn't have the same appeal.
4. Ground. Ground types always come across as quite bulky, which I like. They have good attacks though, and some awesome designs to boot.
5. Dark. I've always been a bit attracted to having a Dark type in my team. I don't know why I like them, I just do.