Originally Posted by Fat NidokingKing
Mareep for High Tier:
Mareep is available early on, but is difficult to get on crystal due to needing to trade.
Great special attack and usable speed.
Special punchs for bonus type coverage, lack of ice punch does hurt it though.
Can easily deal good amounts of damage and take a hit or two, but no more than that.
Electric STAB is amazing with approx. half of the gym leaders having at least one pokemon weak to electric. Not many good resistances sadly (if any).
Matchups are pretty much Falkner, Bugsy, Chuck, Pryce, Will, Lance, Blue, Red, Misty, Brock's fossils, and Janine.
You're overestimating Mareep's Speed and underestimating its bulk. Even as an Ampharos, most things will outspeed it. It underspeeds most fully evolved Pokemon and a lot of the things it doesn't underspeed it has no business fighting.
Not even frail Pokemon will get 2HKO'd by most attacks in GSC. As long as you don't make Ampharos fight something like Rhydon, it will likely be able to tank a few hits. 90 / 75 / 90 bulk is good, after all.
Mention that while the Electric-type doesn't resist much, it is only weak to one type: Ground; as such, Ampharos has a good, neutral match-up against most Pokemon.
Mention that Mareep is pretty strong at the beginning, and then starts to become weaker as time goes on. It becomes fairly useless after a while, until it finally evolves into Ampharos and immediately gets ThunderPunch. Ampharos also gets Fire Punch which is nice. Mareep's really late evolution is annoying, especially when it only has Thunder Shock as a damaging move until it finally evolves, so that should be noted. It's a pretty good Thunder Wave user, though.
Also, @Perfect Cell - it doesn't matter if Misdreavus has a good match-up against Whitney; you can't get Misdreavus before Whitney other than by trading.
I'm not a GSC expert, and in the end, QC's decisions are more important than mine as they have more experience, but I hope my insight helped.
Edit: Right, ThunderPunch TM is available really early in the game.