Season 1 is my favorite by far, but Season 7 is a very close second. The Season 7 Finale was incredible and tied everything together in a brilliant cohesive narrative that nopony had any right to believe could be accomplished. It gave answers to questions that went un-answered for years, completed Starlight's character arc, had the most profound worldbuilding in the show...
And that's only the season's finale. Season 7 also gave us Perfect Pear<3, To Change A Changeling, Triple Threat, Once Upon A Zeppelin and a host of great Glimmer episodes (A Royal Problem, Uncommon Bond, All Bottled Up).