Best Bonus Mode in a Game?

[Proteus214]

Just about anything in a Timesplitters title. The single player mode is just icing on the cake in a game like that.

 

[hazabaza1]

Does Horde mode count?
If it does, /thread.

 

[madman485]

Pokewalker
eternal mode in katamari (also, in the original when you get to roll up countries during the credits)
oh, and the bonus games when you lose in every katamari after the original.

 

[SirNerd]

The little minigame in Snake Eater where you're in a nightmare, killing zombies by ripping them open with swords.

 

[Ultress]

If it count's the challenge/exclusive maps from Portal:Still Alive, If it doesn't either Sisters Mode in Portrait of Ruin, it was basically icy genocide but still pretty fun or this

Mercenaries in the Resident Evil series. Made even better by RE5 which introduced co-op mercenaries. And Barry Burton as playable.

I absolutly loved slaughtering the hell out of things with Krauser's Arm.

 

[Marter]

Time Trials from Mirror's Edge. So frustrating, yet so rewarding.