Mine HAS to be BB FB Trent Williams.....you can give him HB as a secondary position and if you put bulldozer on him, dude is impossible to bring down. I thought Alstott, Henry, Vita, etc, were tough to tackle....but Trent with 90 speed is deceptively quick and insanely powerful. He's tackled by the first tackler maybe 10% of the time? Probably less. And he can freakin' catch out of the backfield too. Low-key an insane card.
Maybe my second favorite card (and I have a 99 O and 99 D) is BB Brandon Marshall. I sometimes user him and MAN is he fun.
Some of these BB cards have been really fun to play with actually.