To be honest I've never seen any truly terrible endings to video games. Some of the lesser ones I've seen though are the endings to Banjo Tooie and the ending to The Elder Scrolls IV: Oblivion.
With Banjo Tooie it just felt like Rare had no idea how to end it properly and didn't feel like thinking about it too much. In their defence though, it was a game that didn't really need a great ending.
With Oblivion, it was mostly the fact that it lacked a real final boss. You just fight your way through the streets of the imperial city, which is kinda cool, but when you reach your destination you just get a short clip, the credits roll and that's the end of the game. You don't even have to fight, you can easily just dash your way through the hordes of Daedra to get to the ending. It's a bit anticlimactic. If they had ended the game at the Paradise dungeon I think it would've been much better.