One of the great things about Unreal Tournament is the way you can have several mods active at the same time with the "mutator" interface; you're not limited to one mod at a time like in Half-Life or Quake. Here are some weird combinations of mutators which I've found are quite fun:

Instagib Domination

"Control point is secure." No it isn't...

Target Practice

Low-gravity Instagib or Sniper Arena (on DM-Morpheus, for example)

The Warlord Hunt

Monster Madness with Instagib or Redeemer Arena (using Weapon Arena for the latter), with lots of monsters including at least one Titan and one Warlord (btw, Titans can survive one Instagib shot).

Grr Aargh

Zombie Beatdown (using the CorpseMania game type) with some of the Unreal4Ever weapons (the pitchfork, shotgun, axe and some of the bullet weapons seem especially appropriate)