I recently played at Showdown and given what I see in practice (although some of the options are not very common, I think that this changes should be made:
Mega Venusaur S > A+ (Reason: Flying and Psyuchic being common types together, uses the Mega slot). I still admit that's very good.
Latios A+ > A (Reason: Reduced usage, Aegislash on the top, Dark and Ghost types buffed)
Latias A+ > A For the same reasons than Latios.
Mega Charizard Y A > A+ (Sun is nerfed but Charizard Y is very powerful). If they are evaluating as a pokemon, they should be S (yes, Charizard in S-tier)
Dragonite A > A+ (Multiscale with reliable recovery, a big variety of sets (offensive, tank, wall DD, mixed), various viable items, less Stealth Rocks)
Kyurem-B: A > A+: 170 Base Attak with the ability of using items freely, Outrage 2HKOing steels, Fusion Bolt hitting many of the fairies, can use special options with Life Orb + 120 base SpA, only pokemon that can use a 2-turn move in OU, very good defenses for a sweeper).
Salamence C+ > B-: It's the best mixed Dragon/Flying with having a useful mixed movepool, various sets viable, two great abilities, faster Dragon Dancer than Dragonite. There are better dragons, but Salamence is not on the level of Diggersby.
Chansey: A > B-: No Leftovers, the Eviolite can be removed with Knock Off, Ghost wall it all day, 404 HP Pokemon also wall it all day, Taunt bait, Setup bait.
In Bold the one that I think needs a change as soon as possible.
I will start doing one nomination:
Roserade for B/B-.
For the following reasons:
-It's the perfect counter of Rotom-W. Natural Cure ures of status, can sleep it before use Volet Switch, 2HKO with Giga Drain, Hydro Pump does pitful damage.
-Has more than acceptable Special Defense. It an sponge various non-suppective attaks with ease.
-Has reovery through Leftovers and Giga Drain. If you want more power, Energy Ball is available.
-Sludge Bomb takes care of the Fairies and Grass types and gives relatively good coverage outside Poison and Steels. It an round overage with Hidden Power Fire, Shadow Ball or Dazzling Gleam, overage that is rare in Grass types.
-It can use various sets with various varations Being a (Toxic) Spiker (with the bonus of absorbing the Toxic ones), being a full sweeper, Being a special tank.
However it has its flaws:
-His coverage is decent but not spetaular and its walled by Steel types that resist Grass.
-Has low physical defense, even with the buff that it got in X/Y.
-90 Speed, while not being bad, it's less than many OU threats , many of them being physical sweepers or have a suppereffective move.
Edit: I think that we need more changes:
Deoxys-S S > A+ Hazards are common but they are not what they were before. Many times don't have hazards and if 10 teams with hazards, 9 run only Stealth Rock. With so much priority in the tier, with anti-lead with Knok Off, with Defog and a new (old) spinner, Excadrill, I thin that deserves A+. Still an excellent lead. Honestly, I'm borderline with it.
Bisharp A > B+ Way buffed but not enough. He has three common weakness , one of them being a x4 one, that even though is not 5th Gen is fairly common in the metagame right now. Is slow for a sweeper, Stiky Web is rare, Intimidate only gives in pratice +1 and Defog can be predicted easily by your opponent. And random drops of Defense/Special Defense are unreliable, nd Bisharp resist many of those moves, and Biusharp isn't absorbing Focus Blast. And Suker Punch an be stalled rather easily (swith, non-attacking moves)