Slowpoke is a huge bother and I really have no idea how you got through HGSS with it. It evolves at level 37, and with the leveling in HGSS that means it won't be evolved until you get to Clair. Also, Slowpoke's Special Attack is a godly Base 40, which means you'll be 3HKOing Golbats and Grimers. How very dominating. Add that to not being able to run away from wildmons. Oh, and Slowpoke has "good" 90/65/40 bulk. Really good, right? Combined with it typing which leaves it weak to Bug, Grass, Dark, Electric, AND Ghost, with three of those being extremely common early-game. It has Base 65 Attack but doesn't get any STAB off that Attack stat until level 32.
In XY yes they are pretty decent thanks to the highly inflated level curve and in RBY it had some use because Psychic-type. Definitely not in HGSS. Unless you have King's Rock+Trade.
Well when you put it that way. I guess I'm just biased because I really like the Slowpoke family and may be going out of my way to make it work -shrug- but I've never had much of an issue with them.
My point was that Slowbro, in my experience, isn't as bad in-game as Blissey.