I put a lot of effort into making this post in the old thread so:
High (with walking... a lot...) / Mid (following a more normal route)
Don't bother with Slowpoke the first time you go to the well. Back track to get a fully evolved king/bro from the well after you get Strength.
Very good stats.
Comes with Surf and Confusion and can use the cheap Ice Punch TM. Headbutt is also useful. Psychic comes really late in the game...
It's a little repetitive with Gyarados, but Slowbro/king have really good typing that gives it excellent match-ups throughout most of the game. It is extremely useful against all of the later half of Johto + Elite four.
High (with walking... a lot...) / Mid (following a more normal route)
Question-- how far are you willing to walk to get a poke? If you don't mind taking it slow... bro/king will really pay off.
Slowpoke is available really early in 2nd Generation, in the slowpoke well which is in the same town as the 2nd Gym; but it's almost pointless to catch it at that point in the game. It comes at a paltry level of 6-8, with only Tackle and it won't get Confusion until level 20. Keep in mind that Slowpoke won't evolve into Slowbro either until level 37, and you'll be hard pressed to find any practical use for it since slowpoke isn't really useful at any of the early gyms (though it can take down some Ghastly/Haunter in Morty's Gym, since only Morty's boss Gengar has Shadow Ball). Anyway, don't bother getting one on your first visit to the well.
The best way to make use of slowpoke is to go to Olivine and get Strength after beating Morty, and walk--yes WALK--all the way back to the slowpoke well. This is a long hike after all, but is a little sped up since you've beaten Sudowoodo by this point. With a bike and some repels, it won't be much of a hassle. Once you get back to the well, you should have Surf and Strength, which will allow you to reach the lower level of the well.
From there, Surf around to pick your poison:
90% Encounter Rate / Level 15-24 + King's Rock + Trade =
(King's Rock is also found on this 2nd Floor, so you can evolve immediately)
10% Encounter Rate / Level 20-25
And then you will finally have your fully evolved bulky Psychic / Water, only slightly under-leveled, but already packing Confusion and Surf for STAB.
Note: If you don't want to walk back to get them before Chuck, you'll have to wait until after him to get Fly to go back to the well. By that point though, you've just missed out on beating one of Slowbro/King's best gym match-ups, and you'll be even more underleveled and wouldn't have gotten to eat up all the experience you would have gained by beating the Black Belt's in Chuck's Gym... really, you want to get it before Chuck's Gym if possible.
Slowking and Slowbro have very good stats, with their 95HP + 110/80 Def/Sp.Def, and are packing a lot of power with 100 Special. Low Speed is not really desirable in-game, but they're both so bulky and powerful that they can make do. Just bring some healing items, as you should because you have so much money.
They have Confusion and Surf for STAB right out-of-the box, and if you were planning to make the massively long treck back to get a Slow-Evo, you probably had the foresight to buy TM 33 in Golden Rod so that you have Ice Punch too! Headbutt is an option if you want a sorta-ok physical move. There are some other TMs that it can use, like Fire Blast / Blizzard / Shadow Ball / Dig, but you probably don't want to use those on bro. Keep in mind, Bro/King won't get Psychic (or any Psychic STAB outside Confusion) until level 54, or reaching Saffron City...
Water is always a great in-game typing, and Psychic is still very nice as Bug- and Dark- attacks are rare. Opponents with Shadow Ball are basically non-existent after you get Slowbro/king. You do somewhat overlap with the Red Gyarados in terms of weaknesses/resistances, but unlike Gyarados, you get Psychic STAB and can make good use of Surf and Ice attacks-- doubling up on Water-types never hurts in GSC (especially since there are 2 Water type HMs you need to move around the map-- Give Surf to Slowbro/king and throw waterfall somewhere on Gyarados).
Chuck: Slowbro/King destroy this gym (especially bro), laughing at Fighting attacks with their massive bulk, blowing enemies away with Confusion, and laughing at anything and everything the gym leader can do (resisting all his attacks except Rage / Fury Swipes, lol)
Jasmine: Obviously, don't send bro/king out against magnemites-- but you should have plenty of teammates that can handle them. Slowbro/king easily take down Steelix, and lol at Iron Tail (again, Slowbro being stronger here).
Pryce: Dewgong's Rest can be annoying... but Slowbro / King shrug off every attack that he has throughout his team, and can look for Confusion hax with Confusion. Slowking is actually the better choice in this gym, taking on more aurora beams.
Clair: Watch out because the 2nd Dragonair has Thunderbolt! They should be able to comfortably take on everything else with Ice Punch though. Kingdra can be a bit annoying. King's Special bulk makes it more useful against this gym.
Will: Ice Punch destroys Xatu, Exegg has no Grass moves and falls to Ice Punch, Jynx's moves are all resisted and you can overwhelm her with Surf or Headbutt, and Slowbro... well, mirror match.
Koga: King/bro can walk through everything he has with Surf and Confusion.
Bruno: Uhh... look out for Thunderpunch...? Ah, who am I kidding, Bro/King destroy Bruno.
Karen: Oh wow! a bad match-up! sorta... King/bro can still be useful here, destroying Gengar (watch out for Destiny Bond...) and being able to beat Murkrow (survive a hit and Ice Punch) or Houndoom (survive a hit and Surf)-- but obviously not both. Umbreon's annoyance and Faint Attack will be too much for King / Bro, and you have no business taking on Vileplume's Petal Dance.
Lance: King is going to struggle a little here, but is still very useful. Slowbro only needs to watch out for the Dragonite with Thunderbolt, but otherwise, it will have no problems with anything on Lance's team, shrugging off physical hits and resisted Special ones while taking his team down with Ice Punch (dragons), Surf (Charizard, Aero), and Confusion (Gyarados).
Lt. Surge: Ok... You've beaten the Elite Four, and you run into the first enemy that's actually a problem...
Sabrina: You resist her, but she don't resist your Surf. This is a good one for Slowking.
Misty: If you went to Saffron, you should have Psychic now, so you'll be able to take out everything except Starmie with ease. You resist all of Starmie's attacks, but it has Recover and Confuse Ray... you're probably best off using something else against Starmie to save yourself the annoyance.
Erika: Ok... here's another one you don't want to deal with... Slowking can be useful with its bulk and Psychic though.
Janine: Psychic DESTROYS her, just like Confusion did against Koga.
Brock: Surf wipes through his whole team, and he can't touch you with anything-- especially if you brought bro. Kabutop's Giga Drain is simply not a threat.
Blaine: Um yeah... Slowking/bro walk through this one too.
Blue: Slowking / Bro should have no problems taking everything down, except the Sunny Day / Solar Beam Exeggcutor. Unless, you're packing Fire Blast and you predict the Sunny Day like a boss.
Red: Erm... Pikachu and Venusaur... ew... The Snorlax also has Amnesia... You have the advantage against Espeon, Charizard and Blastoise though, especially if you have Slowking. Even against Venusaur, there's a fair chance it is going to Sunny Day, so just go for the Psychic (or Fire blast); Slowking can shrug off a Giga Drain and go on the offensive regardless.