My least favorite is Digimon world 4. It felt like a non-digimon game that they crammed digimon in.

I wish they had digivolving as more of a core mechanic; you only digivolved once in that game and when you did you needed to basically restart the game from level 1. I also wish the game/combat wasn't so grindy. Sitting in one spot mashing the attack button was the best strategy most of the time and... just wasn't fun.