It's funny because about 1/3 of the Pokemon mentioned so far are Pokemon ie used in play troughs of my own and they have been fantastic.
Zebstrika, Beartic and samurott are half of my main team on white and as much as I didn't want to evolve dewott, I'm glad I did because he's a powerhouse on my team. Oh, I also used a swoon at on that team until very recently. I got it up to about lvl 60 and got tired of it, but it was solid through the story.
Pokemon I had trouble with were liepard, the monkeys ( not because of evolving early, but because I always had something that outclassed it.) and currently, Zorua. I'm raising one in my black right now and it's just so fragile. Can't wait til it evolves in a few more levels and improves a little.
Also, darumakkas special attack makes me so sad. I had no idea a pokemons sp atk could be so abysmal from such a powerful Pokemon. I gave it flamethrower and flame charge before trading it over and couldn't believe flame charge was outperforming flamethrower so overwhelmingly.