Roserade can double as an status absorber, a cleric, carry double powder and mix all that with the afore mentioned hazard/offensive sets. You can argue that it leads to odd combinations that are somewhat uneffective together, but there is also the fact it has all those abilities and a very different bulk that Froslass (no bulk at all) and Qwilfish (better physical bulk).
The top contenders carry versatility as well (Heracross is also an status absorber, can revenge kill, sweep etc.), but as you mentioned, it is very critical to find out their different movesets in order to counter them properly. For me this is not exclusively a question of how versatile they are, but also of having movesets that counteract other popular strategies. With their kind of versatility it is your opponent who must be guessing how to counter it, not you who has to find the right movement to use, because they are offensive threats. Their brand of versatility cannot be compared with a much less "threatening" pokemon such as Bronzong or Roserade imo.