X and Y were by far the easiest main series game, which kinda took away from the enjoyment (maybe my fault for leaving exp. share on). They got really tired after half of the playthrough. Nevertheless, I still played through it at least once for each version, just because the Pokemon were so cool!
X: Delphox, Aromatiss, Furfrou, Aurorus, Mawile, Hawlucha
Y (rotation): Heliolisk, Gourgeist, Toxicroak, Staraptor, Pyroar, Poliwrath, Florges
Mawile was an absolute beast in X, surprisingly. I've used one in Ruby previously and enjoyed it, but came in especially clutch in these games due to the added fairy typing, more impressive movepool and mega evolution.
I ditched my starter (Chespin) in Y because I got bored of using it and Ifound Gourgeist a much more interesting grass type to use. I mainly switched out between Toxicroak, Pyroar and Florges depending on whatever I needed for the gyms but iirc I left out Florges for the league battle. Staraptor was my favourite to use here cause of Sinnoh nostalgia.