Mantine for Mid
tier (Gold and Crystal). Its lateness and poor level-up movepool are problems but it learns all of the water HMs, making it a neat HM slave, and it has enough power for in-game stuff, which is enough to put it above low.
: Mid-late game. Found by surfing along the routes between Olivine City and Cianwood City. Somewhat rare (10%), but shouldn't take too long to find unless you are really unlucky. It comes at a level in the early 20s so will take a tiny bit of leveling up to match your team.
: Solid overall. 140 Special Defence isn't that useful in game but 80 Special Attack is usable for working through average trainers/rocket grunts. Slightly frail physically but not "Alakazam" frail so it can take physical attacks as long as they aren't ridiculously strong.
: Level up movepool is bad. It only has 8 level up moves and they're all rather weak, making it very reliant on HM/TM usage. However, it learns all of the water HMs, making it a great HM slave. A set of Surf/Icy Wind/Waterfall/Whirlpool gives you a pokemon that can provide HM support that is also a decent battler.
: Not outstanding but good enough. You can teach it Surf as soon as you get it and a 95 STAB attack from 80 Special Attack hits most things hard. It can get through average trainers efficiently.
: STAB on water is cool but the Flying type is useless offensively (it only gets Wing Attack which is too weak). It does have a lot of nice resistances (water, fire, fighting, bug, steel, ground) but watch out for Electric attacks (these aren't that common really).
: It actually walls Chuck but it can't do much back to Poliwrath other than hit it with a weak Take Down (Poliwrath will probably win with Dynamicpunch anyway). Beats Jasmine's Steelix but loses to her Magnemite. Beats Pryce's Piloswine (there are better choices for Dewgong and Seel). Icy Wind can allow it to beat Dragonair but one of them has Thunderbolt (it needs luck to beat Kingdra). Beats a few pokemon at the elite 4 with Surf + Icy Wind (Dragonite, Charizard, Murkrow, Onix, etc.). In Kanto, beats Blaine and some of Brock. It is strong enough to contribute in other fights but the above are its best ones.
Slugma for Bottom
: Very late. Found on Cycling Road in Kanto. Most of the game will be done by the time you reach it.
: Magcargo's stats aren't actually that bad, with high defence and decent special attack and special defence. However, it is slow as hell, the low hp spoils things and the attack stat is disappointing given its mainly physical movepool. Fairly average overall.
: Its level-up movepool is actually cool but it just doesn't have the stats to use Rock Slide or Body Slam. STAB Flamethrower is nice I guess but it isn't really enough to bother with Slugma.
: 80 special attack is ok but it is just too slow. It won't blast through teams when it is slower than everything. It might outspeed stuff at a much lower level (this is Kanto after all) but that isn't exactly a unique niche for Magcargo.
: Good offensively though the Rock typing is wasted on 50 attack. It should go without saying that Rock/Fire is simply awful defensively.
: It can beat Erika (though it doesn't even resist Grass) and deals with Janine's bugs. It can also do ok against Blaine (Rock Slide is weak but it does the job against Fire types at least). Loses to Brock, Misty and probably Sabrina and Lt. Surge (it's too slow for them); can beat some of Blue's pokemon.