We have a large B list. B- candidates that stand out to me:
Sableye. Contrary to what Lolk said, I actually feel this is the worse of the B ranked Pokemon, as it is
very match-up dependent (it is horrible against most full-out special sweepers, being reliant on stuff like Swagger). Being one of the best answers to Mega Kang is not enough when said Pokemon can switch out to a Pokemon that has nothing to fear from Sableye. Not bad enough for C (prankster WoW is awesome), but definitely B-
Infernape
Goodra
110 base special attack =/= 130 base special attack
Latios also fairs well against Mega Zard Y, outspeeding and 2HKOing with LO Psyshock or Psychic, and LO Pyshock deals more damage to Zard Y than how much Latios takes from Modest 252 SpA Overheat
And the Speed difference is a big concern, bigger than Fire Blast. Also, Latios is not the only competition, also Salamence, Hydreigon, and Kyurem-B.
Also, whilst it's great against many special attackers, it's fairs poorly against the commonly used physical Pokemon
Pinsir. It hits very hard with good speed, but it's multitude of weaknesses let it down and ruin its otherwise good bulk. The big factor that makes it B- is that it mandates a mega stone; I can use many other hard-hitters with good speed that don't use a mega stone, like Shyamin-S, Talonflame on the Flying-type side, Latios and Terrakion for general purpose hard-hitting Pokemon, even Deoxys-A can't get 2HKOed thanks to Sash. Compared to competition for the Mega Stone role of hard-hitter, Kang and Mawile don't die nearly as easily, Mega Zard has less weaknesses and spread (and those two weaknesses belong to Pinsir) and Lucario is far more versatile with a much better typing. Flying-STAB Feint is good, but I don't feel a Mega slot is warranted just for it.
I am expecting backlash from
Laga
Abomasnow. It fully deserves the recently earned B rank, but it is quite reliant on TR, has some awful match-ups against very common Pokemon, and has difficulty against Wide Guard users both due to its main move being nuliffied and the fact that the two most common Wide Guard users hit it super-effectively. And it requires a Mega Stone.
Tornadus. For Defiant sets, the loss of item to use its best STAB really lets it down combined with its subpar bulk. Furthermore, it fails to beat Landorus-T even with the Defiant boost, as it cannot OHKO Landorus-T (+1 252 Atk Tornadus Acrobatics (110 BP) vs. 4 HP / 0 Def Landorus-T: 270-318 (84.3 - 99.3%)), and for the biggest insult, the main Lando-T set is Scarf which OHKOes Tornadus with Stone Edge. If Defiant doesn't get any used, I commonly find myself wishing I had another Pokemon in its place.
For Prankster sets, there are better Tailwind users thanks to their much better bulk and resists (they can set it up more than once), and rain sets aren't anything special let alone the fact that they're only worth using on full-blown rain teams and have major competition from the Thundurus forms