Is this for a multiplayer (MP) deck or a duelling deck?
If MP, Demon's comments hit the mark - use lots of cards that affect everyone. If you want to just damage people, with no regard to your own life, run red/black. If you want to stave off the inevitable repercussions of playing global affects, go red/white.
Black helps with cards like Pox, Forsaken Wastes, Lich (theoretically), Urborg Stalker, Subversion, Syphon Soul, Lim-Dul's Hex, the many versions of Pestilence, and the like.
White provides life gain, including Soul Wardens and Congregate. White also provides lots of protection, the simplest example being COP: Red (though in an MP environment, I prefer Story Circle, for its versatility). Playing R/W allows the use of Soltari Guerrillas - one of the more entertaining MP cards.
Don't forget artifacts and lands. Shivan Gorge and Glacial Chasm both work wonders in a deck built to damage everyone, especially the Glacial Chasm. Copper Tablet, Ankh of Mishra, Armageddon Clock, and Razor Pendulum all zap players equally well.
Purple_Jester said:
"The best deck to put Flame Rift into is one where you intend to inflict as much damage in the shortest amount of time." I second this, as targetting everyone at once makes you the target of everyone, at once - which is a bad thing. Start damaging, and do it quickly - hiding behind walls and Glacial Chasm(s) as necessary.
Lastly, I'll echo immortal grave walker. Furnace of Rath. Other useful cards include Sizzle, Acidic Soil, Shard Phoenix, Steam Blast, and Subterranean Spirit.
Cheers