Charizard Y should only use Heat Wave in doubles, since the move hits both opponents. In singles, Fire Blast or Flamethrower are the preferred options:
-Flamethrower/Fire Blast
-Solar Beam
-Focus Blast
-Roost
Nature: Modest/Timid
EVs: 40 HP/252 SAtk/216 Spe
Item: Charizardite Y
or
-Flamethrower
-Solar Beam/Focus Blast
-Will-o-Wisp
-Roost
Nature: Calm/Modest
EVs: 248 HP/96 Def/132 SDef/32 Spe (Calm) or 248 HP/96 Def/132 SAtk/32 Spe (Modest)
Item: Charizardite Y
Generally, a team built around Charizard Y needs the following things (credit goes to Valentine from Smogon, though specific and recommended sets are listed below):
1. Rapid Spin/Defog support: This is crucial because Charizard is 4x weak to Stealth Rock. Both moves clear away entry hazards and the latter affects both sides of the field. Pokemon that can do this for you are Lati@s, Zapdos, Excadrill and Mandibuzz:
Latios:
-Draco Meteor
-Psyshock
-Thunderbolt/Hidden Power (Fighting)/Earthquake
-Defog/Roost/Recover
Nature: Timid/Hasty
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb
Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Healing Wish
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Zapdos:
-Thunderbolt
-Defog
-Roost
-Heat Wave/Toxic/U-turn
Nature: Bold
EVs: 248 HP/168 Def/68 SAtk/20 Spe
Item: Leftovers
Mandibuzz:
-Foul Play
-Roost
-Taunt/Whirlwind
-Defog
Nature: Bold
EVs: 248 HP/136 Def/108 SDef/16 Spe
Item: Leftovers
Ability: Overcoat
Excadrill:
-Earthquake
-Iron Head
-Rapid Spin
-Rock Slide/Stealth Rock
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Leftovers/Air Balloon
Ability: Mold Breaker
or
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe (Choice Scarf) or 120 HP/136 Atk/252 SDef (Assault Vest; Adamant only)
Item: Choice Scarf/Assault Vest
Ability: Mold Breaker
2. Pursuit trapper. Charizard Y is hard-walled by the Latis, as well as Chansey; having one of these helps eliminate these threats. Notable Pursuit users are Tyranitar, Bisharp and Aegislash. Others are Scizor and Escavalier:
Tyranitar:
-Stone Edge
-Crunch
-Pursuit
-Earthquake
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf
Ability: Sand Stream
Bisharp:
-Sucker Punch
-Iron Head
-Knock Off
-Pursuit
Nature: Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Dread Plate/Black Glasses/Life Orb
Ability: Defiant
Aegislash:
-Shadow Ball
-Shadow Sneak/Iron Head/Sacred Sword
-Pursuit
-King's Shield
Nature: Quiet
EVs: 244 HP/12 Def/252 SAtk
Item: Leftovers
or
-Shadow Ball
-Shadow Sneak
-Sacred Sword
-Pursuit
Nature: Hasty/Naive
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb
Scizor:
-U-turn
-Bullet Punch
-Superpower/Brick Break
-Pursuit
Nature: Adamant
EVs: 248 HP/252 SAtk/8 SDef
Item: Choice Band
Ability: Technician
Escavalier:
-Megahorn
-Iron Head
-Drill Run
-Pursuit
Nature: Adamant
EVs: 248 HP/252 Atk/8 SDef
Item: Assault Vest
3. Fighting Pokemon with a Steel resistance: Bisharp is going to be a pain, even if it's weak to Charizard Y because offensive Charizard Y can be slammed with Sucker Punch. These also get rid of Chansey:
Infernape:
-Fire Blast/Flare Blitz
-Close Combat
-Grass Knot
-Thunder Punch/Hidden Power (Ice)/Mach Punch
Nature: Hasty/Naive
EVs: 168 Atk/88 SAtk/252 Spe
Item: Life Orb
Lucario:
-Swords Dance
-Close Combat
-Extreme Speed
-Crunch/Ice Punch/Iron Tail/Earthquake
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
Keldeo:
-Hydro Pump
-Secret Sword
-Hidden Power (Flying)/Icy Wind
-Scald
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
or
-Calm Mind
-Hydro Pump/Scald
-Secret Sword
-Hidden Power (Flying)
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
or
-Substitute
-Hydro Pump/Scald
-Secret Sword
-Endeavor
Nature: Timid
EVs: 8 HP/248 SAtk/252 Spe
Item: Salac Berry
4. Something that deals with Flying spam. Mega Pinsir, Talonflame and/or Staraptor are used together to wear down each other's checks/counters in order to at least let one of them clean up the remaining team late-game. This is devastating, so having something to deal with them is recommended. Zapdos and Tyranitar were touched upon earlier, but Thundurus and Wash Rotom are alternatives:
Thundurus:
-Thunderbolt
-Hidden Power (Ice)
-Psychic/Focus Blast
-Thunder Wave
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb/Leftovers
Ability: Prankster
or
-Nasty Plot
-Thunderbolt
-Hidden Power (Flying/Ice)
-Substitute/Focus Blast/Psychic
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Leftovers/Life Orb
Ability: Prankster
or
-Thunderbolt
-Knock Off
-Superpower/Thunder Wave
-Hidden Power (Ice)
Nature: Hasty/Naive
EVs: 76 Atk/180 SAtk/252 Spe
Item: Life Orb
Wash Rotom:
-Volt Switch
-Hydro Pump
-Will-o-Wisp
-Pain Split/Rest
Nature: Bold/Calm
EVs: 248 HP/216 Def/44 Spe (Bold) or 248 HP/216 SDef/44 Spe (Calm)
Item: Leftovers/Chesto Berry
5. Stealth Rock setter. Stealth Rock is a staple move in competitive play. Usually this move is left to bulky Ground Pokemon so that they can deal with Mega Mawile and Charizard X better:
Therian Landorus:
-Stealth Rock
-Earthquake
-U-turn
-Stone Edge/Knock Off
Nature: Impish
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers/Rocky Helmet
Hippowdon:
-Stealth Rock
-Earthquake
-Whirlwind/Toxic/Stone Edge
-Slack Off
Nature: Impish
EVs: 252 HP/72 Def/184 SDef
Item: Leftovers
Garchomp:
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Life Orb/Focus Sash/Lum Berry
As for your other options, however, Meowstic, Gogoat, Magmortar, Serperior, Pikachu are terrible. They're outclassed by other Pokemon in standard play. However, Ampharos, Houndoom and Banette are only viable in their Mega forms, but your Mega slot is taken up by Charizard.
To let you know, Charizard Y teams tend to be specialized. Each teammate has to focus on eliminating any threats to Charizard before it sweeps.