Ah, someone looking at SD Victreebel.
Basically, that thing's only counter is Skuntank. Everything else, even if it KO'ed it, is crippled like anything by a +2 attack. However, SD Victreebel's weaker with all the Fire types running around now than it was back in the Leafeon days.
I think the set was something like this:
Victreebel@Life Orb
[Attack +, Sp.Def - Nature]
252 Attack, 252 Speed, 4 HP
-Swords Dance
-Leaf Blade
-Sucker Punch
- Sludge Bomb
Even with 0 EV's, Sludge Bomb was a clean OHKO on Leafeon, who was the teir's physical wall at that time, and NEVER saw it coming on a SD set. Leaf Blade decimated anything slower than Victreebel, and Sucker Punch killed more or less anything faster. All you had to do was switch in on something like Quagsire, which was popular at the time.
And, about the varaity in NU, NU has FAR more than 20 viable pokemon. It's just that due to the small playerbase, what works for one person, is often copied, improved, and countered.
Just off the top of my head:
Gligar, Sandslash, Quagsire, Gastrodon, Typhlosion, Charizard, Magmortar, Ninetails, Victreebel, Dodrio, Medicham, Primeape, Espeon, Skuntank, Manetric, Meganium, Floatzel, Quilfish, Golduck [Countering Weather is nice], Eggy [I fail with his name...], Nidoqueen, Porygon2, Articuno, Ampheros, Cactrune, Wallrein, Dusclops, Driftblim, Marowak, Gardevoir, Hitmonchan, Jumpluff, Kangashkan, Lapras, Likylicky, Tauros, Zangoose.
I'm sure there's more too.