While certainly not high teir, I feel like I should argue a few points.I used Scolipede my first playthrough, only because I happened to find a shiny Venipede.
The Venipede phase is almost pure babying. It can finally start to hold its own when it gets access to STAB moves beyond Poison Sting, but considering the is the period of game before Exp Share and Lucky Egg, I found it fairly annoying.
Whirlipede is just meh. Sure, STAB Poison Tail/Bug Bite with decent defenses, but it can hold its own and not much past that.
Scolipede doesn't come into its own until Megahorn, at which point it finally becomes decent. Before that you're working with low BP Moves (Poison Tail, Bug Bite, Bulldoze, etc) with mediocre Attack. It's great against random stuff at this point, but from my experience never truly excelled. It's a bit of a one-trick pony, a fast STAB Megahorn, but it's a damn good trick. It's not powerful enough to deal with the E4 though, and can die a bit fast. I think mine actually got outsped by the Hydreigon. It gets Swords Dance to set up, as well as EQ, but only after the E4.
It's decent enough to get away with if you really like it, but I was a bit underwhelmed with mine. I wouldn't put it past mid, due to a decent period of mediocrity, needing a few levels of babying when it's at its most obnoxious, and underwhelming performance against the E4. STAB Megahorn is strong and all, but it doesn't really have much past that.
When I used Ventipede, I hardly had to baby it. Ventipede x4 Resists Grass. Pinwheel Forest houses Petlil on White, and Cottonee in Black. Both are hit SE by Poision Sting.
Not to mention the good ol' bait and switch on Audino. That really raises levels.
After learning Poision Tail, Ventipede can more than care for itself. Whirlipede was one of my best team members, while I had it, and Scolipede was awesome up until... just after the 7th Gym, where it started to slow down a little. And, by slow down, I mean not OHKO everything. It still outsped everything. I had no issues with lack of Megahorn. In fact, I never even taught mine Bulldoze...