16 slots, any of the 722 pokemon. Make it happen.

Personally, here's what I'd go for:
>Raichu
>Machamp
>Hitmonchan
>Hitmonlee
>Hitmontop
>Blaziken
>Gardevoir
>Breloom
>Hariyama
>Lopunny
>Lucario
>Throh
>Sawk
>Mienshao
>Chesnaught
>Hawlucha

Wide variety of different fighting styles - which is part of what makes fighting games interesting - with a few "popular" picks mixed in that're also fairly viable.