Yeah, Luxchomp's the most popular. Luxchomp is fast, works with the SP engine, and can board control/snipe. Against Gyarados, as dmaster said, is a hard matchup since Lucario GL in play lets Luxray be able to one-shot Gyarados every turn. However, with Gyarados' high damage and insane recovery, Gyarados can one-shot Luxray back.
Rotations occur every September 1st, but the sets that are being rotated out are announced usually in June.