Can't decide. I know what Pokemon the guy gives you, but deciding is hard, you know. For Pokemon black.
There is no "best" fossil. It is purely opinion based. However, if you mean which Pokemon is the best stat wise, the Old Amber, which is revived into Aerodactyl, is the best, due to the fact that Aerodactyl has a base stat total of 515, whereas every other fossil Pokemon given has a base stat total of 495. However, if you mean which has the best typing, Cradily, which evolves from Lileep (revived from Root fossil) and Armaldo, which evolves from Anorith (revived from Claw fossil) are the best, both having only three weaknesses.
Hope I helped!