1. Rock.
Its many weaknessess but powerful attacks with low accuracy make it the ultimate challenge and a good reason to rage quit. Ryperior, Aggron, Gigalith...
2. Ice.
Same reason as Rock, but, since it includes HAX, I don't like it that much. Jynx, Avalugg, Lapras...
3. Ground.
StoneQuake. It is a very craft type to use defensively, and offesively it is reguarding. Ryperior, Dugtrio, Krookodile...
4. Poison.
A great defensive typing, often not used, but useful for trolling and, maybe, stalling. Muk, Weegee Weezing, Tentacruel...
5. Normal.
It has a lot of moves to choose, and it can be combined with many other types. Slaking, Blissey, Heliolisk...