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I'm going to say Kingdra. His stat coverage is above average across the board, and he's only got 2 weaknesses. Both of which are uncommon types.
If that's not good enough, I'll say Rampardos. His base attack stat is #2 on the chart for highest attack stat of any pokemon currently in existence...
Umbreon. Mostly because he's a very good defender, and he looks FRIGGIN' AWESOME.
But I could be biased, because dark-type is my favorite pokemon type. Or at least it was until Generation 6 ruined it with Fairy.