Oracle of Ages is better than Seasons, though both are good games. OoA is my 3rd favorite Zelda game, OoS is my 9th favorite Zelda game.
Though these games are similar, I do not understand why they are lumped together on most rankings. They should be treated as connected but separate games, not like Pokemon releases with only slight differences. As many know, OoA focuses on puzzles and OoS focuses on combat. This makes OoS a rather traditionally good 2D Zelda game, but it makes OoA the most intricate and well-designed 2D game in the series.
The story is better in OoA than OoS (Labrynna is an organized kingdom that has fallen because of Veran, not a disorganized cluster of unrelated locations with unclear leadership and vague references to Zelda 1), the characters are stronger (Ralph is better than Rosa, Nayru is better than Din, Veran is better than Onox, the Labrynna Maku Tree is better than the Holodrum Maku Tree, Ambi is better than Cap'n), the dungeons are better (the Tarm Ruin is the only stand-out OoS dungeon (and maybe Explorer's Crypt and S&S Maze in a bubble), whereas the Skull Dungeon is the only notably uninteresting OoA dungeon), the overworld is more engaging because the time travel differences are complicated and fun to puzzle through, where the season differences are tedious and largely uncomplicated, the puzzle items are more interesting and the original bosses are better than rehashes, and the Zora, Goron and Tokay settlements are fun to navigate, while the Sunken City and disconnected Subrosia provide momentary intrigue but little cohesion.
The Zelda Donkey Kong game in Black Tower is better than Zelda in OoS, the trading quest is more interesting in OoA, Tingle, the Happy Mask Salesman, the Postman, Toilet Hand and Jabu-Jabu are more exciting than Malon/Talon and Biggoron, OoA Impa actually has relevance, the fairy woods is much less obnoxious than the Subrosian stealth mission, Ages has better music.
Seasons has two things that are better: Linked Game integration (the Queen Ambi reveal and the Master Sword are better in Seasons if played after Ages) and the Magnetic Glove is a puzzle item and should be in Ages, not Seasons. The different gems leading to Tarm Ruins are also interesting, but not anymore interesting than the buildup to Crescent Island, the Ancient Tomb, Jabu-Jabu's Belly or Rolling Ridge.
To be clear, Oracle of Seasons does not do any of these things badly: the game is very enjoyable and one of Zelda's many must-play games. It just isn't in the same league as its counterpart OoA.