Mew need I say more it can learn nearly any move it wants that voice is so freaking adorable. As for Pikachu sorry but I have kind of gotten a bit tired of it no offense little buddy but it's so overrated though in the beginning I loved it but as the series progressed it just basically overtook the franchise I blame the anime all the way.
To be honest, I think neither is cuter than the other.
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Mew. It's pretty darned adorable for a pink cat fetus thing.
I picked neither since there's one Pokemon I can say is cuter than both and that is SHINX <3
Claimed Monferno on 12/8/13
The cat fetus is much cuter than the mouse, and less over-exposed to boot.