I forget my Pokemon Y team but I can tell you my Main Story AS team:
Sceptile - I usually choose fire types and I liked Torchic, but I just think Blaziken was too overrated and anyway, I love Pokemon that become dragon types when they Mega Evolve. It did prove useful with X-Scissor, Mega Drain, and Dragon Pulse. Plus, it helps catch legendaries with its False Swipe.
Linoone - Proved as a very useful HM slave. Plus having it know pin missle during the main storyline proved it kinda useful in battle.
Mightyena - For some reason, I usually always use first route Pokemon. Although, I didn't use Bunnelby or Zigzagoon in Y. But Mightyena proved to be useful. Took down Psychics like a pro with Assurance and immunity. Plus, it had an array of types for moves due to it knowing lots of fang moves. Thunder Fang proved super useful.
Beautifly - This one I caught as a Silcoon because Wurmples I get always usually become a Dustox, and I hate Dustox. Beautifly proved useful with Silver Wind and Gigs Drain, although I rarely ever use this one in battle. I used it mostly to light up caves.
Swellow - My flyer that proved useful due to Quick Attack, Aerial Ace, and Fly. I've noticed throughout the games I've played with HMs in them that Fly is one of the greatest flying type moves there is. Also, this one created Secret Bases for me.
Latias - My main mega up until the delta episode. I brought it on the team because getting free legendaries is great, especially if they come with a Mega Stone. Mist Ball proved super useful, so has its Dragon Pulse. This one was also taught Dive and Waterfall.
Now for Post Game, my team became this:
Sceptile
Mightyena
Kyogre - Replaced Beautifly since Beautifly didn't have a use. Kyogre also helped with the E4. Origin Pulse was powerful, plus Ice Beam was also great. It also helped heal with Aqua Ring. Plus, Sheer Cold was great for powerful opponents. This pokemon is able to Primal Evolve.
Rayquaza - Replaced Linoone since I didn't really need a HM slave anymore after the Delta Episode. RAYQUAZA IS A SUPERPOWER!!!! I mean it. It one-hit ko'ed pretty much everything, it even one hitted Deoxys!!!! This is now my new main Mega. It destroys stuff with Dragon Accent and Outrage. I also put in Flamethrower for type coverage and Extreem Speed for a speedy attack.
Dodrio (Nicknamed Dorothy) - So I was trying to capture Pokenav only Pokemon like Buneary in the Safari Zone and I ended up finding a Shiny Doduo! I nicknamed it because I always nickname pokemon I use in contests and started doing Clever contests with it. It was also useful for battle. Since it replaced Swellow I taught it fly, the best move it has. Otherwise, it knows roost, pursuit, and toxic. Toxic isn't as useful as I thought, roost helps a lot, and Pursuit I havent used to attack pokemon being returned so hasn't been useful yet. It also tanks through Sandstorms and Hail with Safety Goggles.
Ampharos - Replaced Latias and is my favorite Pokemon of all time. I got it in the GTS as a French Mareep for the Pokedex, but then I decided why not raise one of my all time favorites. Already Lv100, it has helped evolve my Pokedex filling pokemon. I also use its mega (why I like Ampharos so much) whenever I either don't have Rayquaza, doing stuff at the Battle Resort, or fighting against anyone who plays X or Y. Proves useful with a Rain Dance Thunder combo, Dragon Pulse, and Iron Tail which is for type coverage as a Mega.
Edit:
I just figured out what my Y playthrough team was. It was:
Delphox
Talonflame
Yveltal
Pikachu - First Playthrough, got the idea from the anime
Krookodile - Also got the idea from the anime
Gurdurr - I probably tried evolving it (This was before I got access to trading)