i use to have an obsession with theme teams (see: spl ash team against mr.e). i've actually made mono-teams for each type in the past, and quite frankly it's non-representative of the following list, apart from #1.
1. normal - again the best of the best is normal. honestly given how good lax is he alone should be good enough to take this spot. but then you add stuff like miltank/blissey who are game changers in their own rights. body slam is every bit as good, and the hyper beam dominance is replaced by a buffed up double-edge and no fuss attack in return/frustration. there's a reason normal resists are mandatory
gap - as with viability list, i feel like there's a gap here.
2. electric - again, when you think of gsc you think of "snorlax and the electrics". when combined, they're every bit as centralizing as snorlax itself, defensively it's the best special typing in the game, with no weaknesses at all. hits for se against water and flying, who in their own rights are extremely popular types (albeit flying is really just skarmory and the odd charizard for all intents and purposes). jolteon's pretty good too. thunder wave is a bit of a lost move on electrics itself but the later-era of gsc brought about the popularity of thunder to spread some para-love. thunderbeam is arguably more popular than boltbeam on the likes of nidoking/dnite. the blizzard nerf makes thunder one of the best 120bp special attacks in the game (vhspa is the term i believe?). zap/kou is carrying this tier.
gap - again another gap between the two "mandatory" tiers onto more conventional levels of power.
3. ground - earthquake is probably the best "splashable" attack in the game. given the pretty big powercreep between the top two types and the rest of the "mortal" types, ground is made much more powerful with the prominence of electric mons. given the current hp legend meta, zapdos/raikou is literally on every team. without ground types, they're free to throw out thunders. having a ground type, despite not directly being a counter to zapdos persay, puts a slight halt to that. having a raikou/blissey is not enough to stop the thunder spam, so if you want to bring in a machamp or clefable or something, you want to do it against an hp ice. having ground types are the only way of forcing that. moreover, ground pokemon are pretty damn good. nidoking, steelix, and marowak are well within the conversation of best regular ou pokemon. ground types are absolutely mandatory in team building for me. eq is simply amazing coverage, and is the main reason it's the best of the three for me.
4/5. steel - defensive typing in its purest form. a normal resist is absolutely mandatory (ghosts don't count), but unlike ground types there's a substitute for its role. forretress, steelix, and skarm. the "other" spiker and the two best physical phazers. there's simply no offensive presence to speak of; i don't have a single team with iron tail/hp steel. defensively though, it's what totally shaped gsc.
5/4. water - "bulky waters" is still a thing, despite suicune having fallen off. water gets the nod for me over steel types primarily because it has a usable attack. starmie, suicune, vaporeon, and cloyster are your main players here: the best spiker, the best spinner, the best special sweeper, and the best mixed sweeper wall. suicune's fallen off recently given the rise in popularity of explosion teams, but that's actually pretty unfounded. sure you can't run suicune ON one, but suicune actually does pretty well against explosion teams. the winrate speaks for itself. surf is a usable attack, but nowhere near as good as earthquake. hp is alright too, but really only on vaporeon. water as an offensive typing is pretty mediocre.
water vs steel is totally interesting. when you think of water, you think of the water POKEMON. when you think of steel, you think of the steel TYPING. it's the one type whose association is purely on its type. you think forretress you think steel, you think skarm you think steel, and STEELix speaks for itself. no other type has the same connotation, but how much credit do you give it though? where do you draw the line? water pokemon are better than steel pokemon, water attacks are better, but steel just feels like it has a bigger impact. i could be looking too much into this though.
gap - just wanted to discuss the top 5 for now. the other typing becomes pretty average. fighting/flying both make a pretty good showing, flying type pokemon are actually especially strong, albeit not due to the flying typing -- throwback to rby. rocks are the other normal resist, but in an hp legend meta that's basically just ttar. poison pokemon are better now, and it has some defensive merit given sludge bomb/megahorn/cross chop addiiton (gengar/nido are solidly in ou, and tenta/muk are creme of the crop bl pokemon). psychic is ok, but personally if i could make egg pure grass i would. bug is ok, sorta comparable to dark for me. decent typing, but i've been perfectly fine building teams without bug/dark pokemon and bug/dark attacks.