Exception of Dragonite? Weavile is WAY better vs Drago because of Ice Punch that OHKOes
even with Multiscale.
Couple of Calcs to support Weavile to B+:
1) Ferrothorn
252 Atk Life Orb Weavile Low Kick (100 BP) vs. 252 HP / 88+ Def Ferrothorn: 179-213 (50.85 - 60.51%) -- guaranteed 2HKO after Stealth Rock
2) Heatran:
252 Atk Life Orb Weavile Low Kick (120 BP) vs. 0 HP / 0 Def Heatran: 307-361 (95.04 - 111.76%) -- guaranteed OHKO after Stealth Rock
3)Tyranitar:
252 Atk Life Orb Weavile Low Kick (120 BP) vs. 252 HP / 252+ Def Tyranitar: 437-515 (108.16 - 127.47%) -- guaranteed OHKO
4) Breloom:
252 Atk Life Orb Weavile Ice Punch vs. 4 HP / 0 Def Breloom: 361-429 (137.78 - 163.74%) -- guaranteed OHKO
5) Latios:
252 Atk Life Orb Weavile Ice Punch vs. 0 HP / 0 Def Latios: 361-429 (119.93 - 142.52%) -- guaranteed OHKO
Out of the top ten mons in terms of usage Weavile outspeeds and beats Ferro, Dragonite, Heatran, Ttar, Breloom and Latios: a majority of the most used mons in the Top Ten(
http://www.smogon.com/forums/showthread.php?t=3482967)
It is for this reason only that Weavile should be B+
Roserade for C-Rank
To qualify as a good sweeper, I believe that the following are certain qualities necessary in the current metagame:
a) High speed OR good priority: With Roserade, while 90 base speed isn't bad, most of OU outspeeds it and Roserade has no form of priority to beat mons that are naturally faster than it
b) Good coverage: While the grass type is good as coverage, poison just sucks as an offensive typing. And together combined, looking at the LARGE NUMBERS of Dragon and Steel types, Roserade can't offer this
c) High attacking stats: Roserade does qualify here: 125 SpA is quite high
d) Ability to tank a hit: Most sweepers can tank EITHER special OR physical hits if not both. Roserade can't do either in OU. With 60/55 for physical, anything neutral does MASSIVE damage. More importantly though, a lot of the special attackers in OU happen to be Psychic types like Latios, Starmie and Alakazam which makes things bad because even if 105 SpD is good, those mons will still OHKO.