Okay so clearly I played Y a little different to everyone else... I rotated like 20 Pokemon when I got to the Elite Four. It was mostly because it's so easy for your Pokemon to level up in Y thanks to the lengthy gameplay between the first and second badges, the EXP share and the relatively low levels of gyms, trainers and elite four. It meant I was able to rotate heaps of Pokemon and never fall behind. So, read and weep...
Mew Lv 65 Synchronise
Calm Mind / Metronome / Dazzling Gleam / Psychic
Someone traded this to me for a Lv30 Floette, wtf?! It's also the closest thing I have to a Fairy type, with Dazzling Gleam and a Pixie Plate. Calm Mind when it comes in because of good speed and defence stats and Metronome just for shits and gigs because it's cool and you can risk it with a Mew.
Abomasnow Lv63 Snow Warning (Mega)
Blizzard / Ice Shard / Ice Punch / Wood Hammer
Annoying that Ice Punch is the most powerful physical Ice move, as Abomasnow's attack is better than its special attack, but this is a cool Pokemon as long as you don't use it against a fire-type. Its mega type looks wicked.
Blastoise Lv64 Torrent (Mega)
Iron Defense / Surf / Skull Bash / Aqua Tail
GREAT all-round stats. Defence boosts turn it into a physical wall and Surf is great as we all know.
Aegislash Lv64 Stance Change
Retaliate / Iron Head / King's Shield / Shadow Ball
Awesome to switch into with Retaliate and King's Shield against physical types. I love the strategy of using Aegislash. Fun.
Gogoat Lv63 Sap Sipper
Horn Leech / Earthquake / Leaf Blade / Energy Ball
One of my fav Pokemon of this gen. Looks super cool and boasts pretty great attack stats. Earthquake to cover some weaknesses.
Scrafty Lv61 Moxie
High Jump Kick / Crunch / Bulk Up / Brick Break
Another of my favourites and if I was to cut this down to six Pokemon, Scrafty would be in it. Terrific defences and good Atk stat as well. Lead with a Bulk Up and then it deals huge damage with High Jump Kick and stab on all its moves.
Avalugg Lv61 Own Tempo
Double-Edge / Rest / Skull Bash / Avalanche
About ready to dump Avalugg. Better early on when you face less Pokemon with special moves, and fun to play strategy with but it's Sp Def stat is rubbish so cool against physical Pokemon but pretty useless otherwise.
Absol Lv63 Super Luck (Mega)
Swords Dance / X-Scissor / Night Slash / Future Sight
Another of my favourites. Wish it had better defences.
Rhyperior Lv62 Solid Rock
Stone Edge / Earthquake / Rock Polish / Drill Run
Got the perfect set goin on this one I reckon. Rock Polish allows it to hit first against most Pokemon and both Stone Edge and Earthquake are powerful. I might replace Drill Run with an electric move to cover its weaknesses.
Talonflame Lv65 Flame Body
Overheat / Steel Wing / Fly / Aerial Ace
Having a fire Pokemon this fast is so good. Love Talonflame, the grey underbody is a bit ugly, but a great Pokemon.
Heracross Lv61 Guts (Mega)
Take Down / Megahorn / Close Combat / Swords Dance
Another favourite, running all out on Atk.
Magnezone Lv64 Sturdy
Thunder / Flash Cannon / Metal Sound / Discharge
Sturdy means it can survive an Earthquake, and it has the most resistances of any type. Aimed with Metal Sound and two stab Sp Atk moves of both its types, this is a great Pokemon.
Yveltal Lv64 Dark Aura
Focus Blast / Oblivion Wing / Hurricane / Dark Pulse
Obviously a beast. Focus Blast to cover two of its weaknesses. Oblivion Wing might be the best move the game has ever seen, I love it so much and it looks cool as too.
Malamar Lv63 Contrary
Foul Play / Swagger / Psycho Cut / Hypnosis
Another favourite. I love using Swagger/Foul Play and Contrary is a fucking wicked ability. Good Pokemon to start with.
Lapras Lv63 Water Absorb
Surf / Blizzard / Ice Beam / Brine
Yet another favourite. Has great all-round stats, save for being a bit slow, so can survive most hits. Surf, Ice Beam and Blizzard all capable of big damage and then Brine finishes them off strong.
Gengar Lv64 Levitate (Mega)
Toxic / Shadow Ball / Curse / Hex
Loses Levitate when it megaevolves annoyingly, but a dope Pokemon as we all know well. I love using Toxic/Hex, Shadow Ball fucking rules thanks to its high Sp Atk, and there's Curse for when you're up shit creek without a paddle
Dragonite Lv63 Inner Focus
Dragon Dance / Draco Meteor / Dragon Claw / Flamethrower
Might replace Dragon Dance... what do you reckon? Maybe Calm Mind... since Dragon Claw already gets STAB. Not quite as good as I remembered, and Dragonair might be the best looking Pokemon in the game. But still damn good.
Delphox Lv63 Blaze
Flamethrower / Psychic / Fire Spin / Fire Blast
My starter. Fire Spin/Flamethrower most often, Fire Blast for the big jobs, and having stab Psychic rules.
Raichu Lv64 Static
Light Screen / Nuzzle / Wild Charge / Electro Ball
Nuzzle>Electro Ball is fucking so good, particularly when thanks to its high Speed Raichu is faster than most already. Wild Charge to take advantage of its high Attack. Not sold on Light Screen. Thinking of Double Team maybe... or Reflect, as its Defence is lowest stat.
Lucario Lv60 Steadfast (Mega)
Flash Cannon / Calm Mind / Aura Sphere / Close Combat
Another favourite. This is a perfect set if you ask me. Calm Mind to start with, a fighting and a steel stab Sp Atk move, great stats, and the awesome Close Combat.
And I have since added...
Zygarde Lv70 Aura Break
Crunch / Land's Wrath / Camouflage / Dragon Pulse
Open to suggestions on this bad boy.
Mewtwo Lv70 Pressure (Mega)
Light Screen / Reflect / Psychic / Focus Blast
Jesus christ, how good is this Pokemon. Huge all-round stats and insane Sp Atk. You have time to set up the barrier moves, and Focus Blast counters Dark and Steel types.
I've also had Pokemon like Bisharp, Salamence, Drapion and others but I've given up on them. Salamence because its moves were a mess, but I will keep it and transfer to Omega Ruby for the Mega Stone, and Drapion, because I was trying to run a stall/setup set on it (Toxic/Toxic Spikes/Infestation/Crunch) because it only has one weakness but the setups take too long and it doesn't have good special defence so often gets knocked out pretty quick.