Personally, in OU I like using Shaymin, as it can help lure in and deal with Scizor, once they realise it's SubSeed, and then their Scizor's gone for the rest of the match. It also has powerful moves that can do damage, while it can outlast Dragonite, as well as many others.
I liked running SubSeed Venusaur in the Sun at one point. First turn you'd Sleep Powder, then Sub as they switch out, and then set a Leech on them. It could deal with any opposing grass types with a sun boosted Hidden Power [Fire], letting it lure in certain threats such as Ferrothorn, who thought it was safe to wall.
Other SubSeeders don't catch my eye because they have no interesting features. Breloom is the only one other than those two (and personally I don't think Venusaur is good, it's just fun) that seems remotely interesting, as it can actually KO some stuff, unlike Whimsicott.