I have to agree that Houndour is a B or higher, especially if one's starter isn't Chimchar; even though it's frail, caught mid-game, and requires the Flamethrower/FireBlast TM (and PP Ups with FB), it combines two mostly-unrelated STAB types (Fire and Dark), which can be really useful in hitting as much stuff as possible, and it's immune to Psychic (and Fire, if you catch a Flash Fire 'Dour). With Faint Attack, it totally dominates Candice's Froslass.
Rotom - S - Even though it only has Thunderbolt/Discharge/Shadow Ball without the Secret Key, its typing and ability means that it comes close to walling Maylene's entire gym, along with most Ground/Normal types (avoid Night Slashing Normals). Electric/Ghost allows you to hit many different types, with tons of resistances/immunities. On top of that, it pairs fabulously with Torterra during in-game double battles, allowing you to spam Discharge/Earthquake to your heart's content :naughty:.
Buizel - B - Wicked fast, decent Atk for firing off Aqua Jet/Ice Punch/Crunch, but doesn't get Waterfall until after 8th badge, and it's not sturdy at all. (Personal aside: Floatzel's cry gets really annoying after awhile)
Snover - C - Decent STABs; gets both Wood Hammer and Blizzard by level 41. Unfortunately, caught VERY late, gets outsped by Cynthia's Garchomp, and the other team members won't like the hail at all.
Rhyhorn - C - If you're patient enough to catch it mid-game, use Earthquake TM on it, and wait ~15 levels for it to evolve, Rhydon/Rhyperior can outperform Graveler/Golem from that point forward. Unfortunately, Rock/Ground loses its use late-game, although it rips Volkner a new one, outside of Raichu's Focus Blast.
Skorupi - D - No one wants to sit around in the Great Marsh looking for it. Plus, its only value is in Drapion's Crunch, the elemental fangs, and Psychic immunity.
Croagunk - C - Poison STAB is almost useless; if it can avoid Psychic/Ground moves, Revenge MIGHT be of use.
Eevee/Espeon - C - Eevee, as a movepool-barren Normal-type, is dead weight; lugging around Eevee trying to bolster its happiness is unfortunate, but Espeon is fast, with great SpA with which to beat down Team Galactic. Almost requires Psychic TM, since Psybeam doesn't cut it after awhile, and its special movepool is restricted to Psychic/Swift/Shadow Ball.
Eevee/Umbreon - D - See Espeon regarding happiness; it can't kill anything not weak to Dark, especially without Payback.