Powerful, or useful?
Venusaur has exceelnt power, and is usually preffered in competetive battles. However, Blastoise does have certain things (like mirror coat, haze, and rapid spin), that venusaur lacks. My vote:
Alakazam > Kadabra, but Pidgeot just sucks. 3 pokes outclass pidgeot (considering there are only 4, thats bad). The others (in order) are Dodrio/Swellow > Fearow > Pidgeot (Dodrio and Swellow temporarily tie for best normal flying).
This one depends on whats available. Lets go into analysis:
Jolteon has better speed and Special attack, but has severe problems with blissey, and special walls in general.
Raichu would have been better than Pikachu, had it also gotteen the LightBall. However, it didn't. So, Pikachu now sometimes is seen more often than raichu (because of the item). Anyway, it is at least able to damage the over inflated things with high special defense as long as it packs brick break (they always lack defense).
Anyway, if you have Raichu from pokemon box (where you get special moves), it will have Surf, allowing thunder-dance.
That is big, as Surf is typically preffered to the other hidden power types (its a constant battle of coverage v s. power).
My vote rests with Jolteon, though Pikachu @LightBall/Raichu isn't a bad choice.
Heracross hands down. There are other bug/flying pokemon (albeit, this one gets swords dance).
Zapdos > Articuno > Moltres. Zapdos has good movepool (it can use HP Grass as opposed to beoing forced into hp ice due to grasses, because of the 80 power drill peck. Moltres suffers from being a fire type. Articuno suffers from having almost no good movepool (it has a few good moves, though, like ice beam, water pulse, extrasensory, reflect).