Ok, a few more mons:
Mareep: Ok, I have no goddamn clue how you guys ranked this high. I thought to myself that maybe I was incorrect; maybe Ampharos was a really good mon that I wasn't giving enough credit. However, having played with it, I really can't understand how people can rank it high; hell, I have difficulty ranking it mid. Throughout the game, it gets nothing but Electric-type moves, Power Gem, and (later on or early if you tutor it) Signal Beam. Outside of those types, it has nothing, meaning that every single move it has until the end of Victory Road is not over 90 BP. Throughout the game, I was often kind of astounded how weak it could be; Thundershock didn't 2HKO things and then it got Electro Ball, which soon stopped KOing things... and then there's this simply massive period of time where it doesn't get anything but Discharge, stuck with a measly 60 BP move it can't abuse well at all. Its utility moves are nice, sure, but I can't feel that Ampahros is very strong. It's not overwhelmingly bulky, either. I dropped it after the 8th gym. Low, Mid at best.
Clefable: Clefable may not seem like an overwhelmingly strong mon - and it isn't - but it is one of the few Pokemon capable of so easily cheesing most of the game's boss fights. Catch one outside of Giant Chasm (at level 50-52, sheesh) that has Magic Guard and it can sweep Ghetsis, Shauntal, and Iris (it sets up on Hydreigon and Lapras!) with Cosmic Power / Stored Power (obviously, use Body Slam to take out Dark-types). For some reason, Gamefreak enjoyed dumping mons that make the Elite Four easymode in Giant Chasm and Clefable is simply one of those mons. Mid.
Vespiquen: Frankly, I really wanted to like this thing. However, it's not easy. Vespiquen is laden with weaknesses. See a Pokemon you might take out with Acrobatics? Nope, it outspeeds you (this happened frequently with Gurdurr...) and has a super-effective Rock-type move. At times I was impressed by its bulk (living +1 Drilbur Rock Slide in Clay's Gym) and at others less so. It was never particularly strong; for example, it couldn't 2HKO wild Zebstrika with X-scissor when at the same level as they were. Acrobatics is a perk for Vespiquen, but one that it lacks for a good portion of the game; without it, its best STAB is Signal Beam, which is kind of silly. Otherwise, Gamefreak clearly doesn't want players to use Vespiquen at all - the chance of finding a female Combee is about 1.25%. I got lucky finding mine (it only took 20 minutes!) but others may not be equally so. Low, possibly Bottom.
Pidove: Although it was total crap last game, it's very respectable in this one. A set of Return/Fly/Roost/Quick Attack served me very well throughout the game and I was always pretty happy with Unfezant's strength. Quick Attack has a surprisingly great utility, for some reason, and Fly/Return has oddly good coverage, although, obviously, is lacking. It 2HKOes most things, which is respectable. Mid.
Excadrill: Speaking of cheesing most fights, one can do the same with Excadrill. Excadrill has one of (if not the) strongest STAB move you're going to use ingame at level 36. Before that, it has the slightly weaker Dig. Basically, Excadrill sweeps through most things that it touches; once it gets +2, it is a veritable killing machine. Otherwise, I am of firm belief that the Iron Head tutor is made solely for Excadrill and nothing else. It has wonderful Attack, passable Speed, Steel-typing... what's not to love? Top.
Camerupt: Although Camerupt is not the strongest Pokemon, it does have nice mixed offenses and hits quite hard when you need it to. Fire Blast is a great move on any Pokemon that can use it with STAB and Camerupt is no exception. It wishes it was faster; although it does learn Rock Polish, it's inherently very slow. In any case, it's a pretty good Pokemon and not one without its redeeming qualities. Mid.
Spheal: I caught my Spheal at the rather high level of 39, but that isn't overwhelmingly unlikely to occur. Although an Ice-typing is a terrible thing in competitive play, it's a huge benefit for Spheal, which sweeps Drayden's gym with it if you're willing to overlevel it. Although it doesn't get Ice Beam for a while, Aurora Beam does out a sufficient amount of damage until you upgrade. Even though it has little other than its STAB moves - Ice Beam and Surf - those moves carry it through the game pretty well. Ice Beam obviously helps against Iris, too. Mid
Tangela: Although I had a Tangela with Regenerator due to some luck with Hidden Hollows, I don't feel that Regenerator affected my play too much. Tangrowth is a mixed bag, to be frank. On one hand, it has a strong Giga Drain, even as a Tangela. Late in the game, it gets Power Whip, which hits very hard given its base power. It has fantastic physical bulk, too. However, it really lacks moves. Outside of Grass-type moves and Ancientpower, it lacks offense, unless you want to teach it Bulldoze. Leech Seed isn't available and Sleep Powder is inaccurate at times. At times, I felt like I was really running little but a slower Lilligant that always had to take a hit. Low, but an argument could be made for Mid due to its nice bulk and power.