For fire, Typhlosion. I too, think it's a cool-looking fire type. The fire starters are definitely my favorite since all their fully evolved forms look really cool, and they're all really good sweepers.
For Water, it's Blastoise. In my opinion, it's a really good pokemon, even if it's in the UU tier. It may not be the best, but it is a really good Rapid Spinner and a Mixed Wall too.
For Grass, it's Meganium. I really like the way it's designed. And it doesn't have that kind of um... "evil" look to it. I couldn't find a better word, sorry. xD
If we're talking about battling, there is no best pokemon, your pokemon's strength depends mostly on how you use it.