I thought Light might have an Eevee as it has the most strategic value; you can react to any scenario you may end up in with strategic precision when you have so many evolution choices. And Light can adapt: when he has to make that decision, he can ensure it works for all of his plans. And Eevee itself mostly just seems cute and ineffectual, not a heavy hitter, so it's also a disguise.

I think L having a Psyduck would be funny as its perpetual confusion is oddball enough it'd fit with his aesthetic. It's also the opposite of Light: it's a terrible strategic choice, and what great detective would opt to do that?

-timydamonkey