Hard decision.... Usually I like to go with grass or water. Right now my team consists of Fire and Grass pokemon though so maybe those 2 types. Dragon is also really cool ^_^
Hmm...well, the types I use the most are water, psychic, and dragon...I'd probably have to say water, because of the high versatility of dual-types, and a huge moveset...as well as plenty to choose from! (Up until Gen 5, water always had either the biggest or one of the biggest categories of Pokemon)
[font=courier new,courier,monospace]I tend to choose the fire starter in each game until I chose Snivy in B/W. But my favorite type would have to be Psychic.[/font]
I am a very big fan of Poison types. So many people undervalue them because "ground is a common type" and most poison types are weak to ground. Know what else is popular? Fighting types. Who resists fighting types? That's right. Besides that, Crobat, Gengar, Weezing... all of them have a means to avoid ground attack (only 1 if mold breaker is around though) and 2 of them have great means to counter Psychic types too. But my biggest reason I like them? Many of them are purple lol! Yeah, favourite colour!
Personally I feel like saying Dragon is cheating since about half the dragon-types now are Legendaries.
I tend to balance out my team as evenly as possible, but the ones I use most are usually fire or electric types. Out of battling, all of my favorite Pokemon (Magnemite, Skarmory, Bisharp, Scizor, Lucario) are Steel-type, so yeah that.