@TM13, I never said I disagreed with Kadabra being Top. He is probably the best Pokemon in the game (Feraligatr can give him a run for his money, though~) thanks to having the Elemental Punches and the most powerful STAB Psychic in the game.holy crap, i just found out Haunter doesn't learn the TM Punches. seriously??? he's got freaking floating hands, his punches should be awesome.
Abra is Top Tier, with or without trade. Gastly (trade) is High, while Gastly (no trade) might even be Low; personally I'd put him in the Mid Tier because he has niche uses (as mentioned, Miltank and Snorlax, which are among the most annoying Pokemon in the game). Scizor and Steelix are Mid Tier only if you don't consider how annoying it is to get Metal Coat; if you just want to make a note of it under "Availability," I suppose they are considerably better than all the guys listed in the Low Tier.
If we're counting Steelix and Scizor, I honestly don't see what Scizor has over Scyther. Scyther's fast and hits hard. Scizor is slower, but hits harder with its slightly higher Attack stat.
Scizor gets natural Metal Claw and False Swipe, both very nifty moves for in-game. On the other hand, Scyther gets Wing Attack and Slash, while also getting natural False Swipe. I'm curious as to how the added Steel typing could help Scizor get SD boosts off.
Also, the fact you're catching so many Magnemite could be considered tedious. But then, this is the same thing as "oh, lets make sure we can get to Lapras by Friday, so we're going to predict when we'll meet it". To me, that sounds like a lot of work that I wouldn't want to put in if I were playing to just beat the game. I'd rather just grab a Mantine (in GC) or Tentacool and be done with it.
Speaking of Mantine, he's actually pretty good. I'll get to writing a review of him since I use him a lot in my runs.
Mantine Mid Tier
Availability: He's a somewhat rare encounter on the sea routes between Olivine and Cianwood. You'll probably encounter at least one if you don't Repel spam.
Stats: Mantine's got a gargantuan 140 Base Special Defense and a respectable Base 80 Special Attack. Defense and HP are a little low at 70 and 65 respectively, and don't even bother with physical attacks, as it's not killing anything with Base 40 Attack. Speed's kinda mediocre as well at Base 70, but it's enough to get it by.
Movepool: Surf and Blizzard. These are the only two moves that matter, and Mantine uses them pretty darn well. Most of the things Mantine likes to hit use either Special Attacks or are slower than he is, so Mantine can afford to only have a couple of good moves. Yeah, his movepool is pretty barren...
Matchups: Mantine can beat Chuck's Primeape, but can't really harm his Poliwrath. Jasmine's Piloswine, Clair's three Dragonairs, and Pryce's Piloswine also fall to the manta ray. In the E4, Will has trouble actually hurting Mantine thanks to its Special Defense, but can't kill Slowbro. Koga's Ariados, Crobat and Venomoth also fall to Mantine, but Forretress likes to blow up and Muk likes to sit there and take Special hits all day. Bruno's Machamp has Rock Slide, but everyone else dies to Surf. Karen's Houndoom, Umbreon, and (sadly, terrible) Gengar all die to Mantine. The Dragonites outspeed Mantine, but Blizzard sorts them out, and only the Thunder Dragonite really poses a problem. In Kanto, Blaine, Janine, Brock, and part of Blue's team fall to Mantine. Red's Charizard is also no match for Mantine.
Additional Comments: Mantine's problem is a lack of moves. This is one of those things that GSC is famous for. Many Pokemon in this Generation suffer from not having a move they really want. Mantine really wants Ice Beam, but only gets it in Crystal. He's also not as diverse as some of the other Water types (looking at you, Lanturn), and he's pretty rare. All these factors plant him solidly in Mid Tier.