These 2 colors compliment each other beautifully.
Red / White
Mass red damage / Prot-red creatures and other damage prevention
Small efficient critters / small efficient critters
Mass destruction for ground creatures / good flyers
And so many more. I have 3 R/w decks right now (and now that I think about it 3 R/W/G decks also, huh)
Black / Green is a close 2nd.
Green / Blue 3rd
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I like building opposing color decks much more b/c they are usually much more original and versatile.