Sneasel-Sandshrew/Weavile-Sandslash
Alakazam/Feraligatr
Mantine/Gliscor
I don't like to mix evolutionary levels, and I don't like to use legendaries unless the prospects are incredibly awesome, like Charizard/Mewtwo
Did that awhile ago.
More suggestions people.
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"CROSSFIYAAAAAAH!" counter: More times than you've had hot meals and been laid, combined