So far, Crandios/Rampardos have been my favorite fossil Pokemon. It was the first fossil Pokemon that I raised to level one hundred and I loved their designs. I think Archen/Archeops would be in second since I really like their designs and I can see them being pretty useful for my Black 2 team.
Really struggled to choose between Shieldon / Bastiodon and Tirtouga / Carracosta for this one. In the end I went with Tirtouga / Carracosta, because it's the one I've used most recently and it occurred to me more readily than Shieldon / Bastiodon did. Though I think I'm beginning to regret my decision...
I like Tirtouga/Carracosta and Cranidos/Rampardos both are very good. Cranidos was the first fossil pokemon i actually used because even though I did get Anorith in Gen III I never evolved it and everything even though Armaldo looks awesome
I voted for Cranidos/Rampardos, simply because I think their designs are the best among the fossil pokémon, but Tirtouga/Carracosta also hold a special place in my heart, being the first fossil pokémon I actually used in my team (albeit only briefly).
I'm going to have to vote for Aerodactyl, though. Not just because of its awesome design and great name, but because he was my first fossil Pokemon. I get attached to all of the Aerodactyls under my care. It's one of the staples of my team, actually.
Rampardos, Arcehops and Armaldo get honorable mentions for their Attack stats and the fact that I use them quite a lot.. Shame their...everything else is lackluster. I still adore using them, though.