Out of all fossil Pokémon (excluding the ones from X and Y, as they're a little early to judge due to being new), my favorite would have to be Bastiodon. Its head looks like a gigantic shield, which is quite awesome! Thanks to its head, this Pokémon's able to take in a lot of hits without being damaged too much. Not only that, Bastiodon's got a variety of moves in its arsenal as well, most of which you may find to be helpful when battling competitively. Some of these moves are Stealth Rock, Roar, Toxic, and Magic Coat. Stealth Rock gives you the chance to hurt your opponent's Pokémon as soon as they get sent out on the field, making it a bit easier for you to have them taken out. With Roar, you're able to force your opponent to switch their Pokémon if they're a threat to Bastiodon. Toxic allows you to cripple your opponent's walls, as well as some sweepers. And with Magic Coat, you can throw back any non-damaging move that your opponent uses on you. If you have all four of these attacks on your Bastiodon as a complete moveset, then your opponent should be having quite a hard time stopping you. (:
Armaldo is also one of my favorite fossil Pokémon. Its design is pretty nice, and I find its smile to be really adorable! The only thing that I don't like about it is its arms—in my opinion, they're kiiiind of placed in an awkward position. If the arms were a little higher on its body, then it definitely would have looked better. From a competitive standpoint, Armaldo isn't that bad either. Its base Attack stat is one twenty-five, which is very decent. After the use of Swords Dance, this Pokémon could sweep just about anybody!
Anyway, from each region, I like Kabuto, Aerodactyl, Anorith, Cranidos, Archen and Tyrunt. Their offensive nature in battle is what I like about them. I especially like Tyrunt because we finally have a T-rex Pokemon.
I have gone back and forth on picking a favorite starter so many times in the past because they are really all so amazing in each of their own rights, but I will say that my current favorite is Aerodactyl. I have once again fallen in love with this Pokemon because of X&Y. it just looks so badass in it's Mega forme which improves it tremendously. It was already an amazing Pokemon but that has now doubled in my eyes.
As for my least favorite, I will say Bastiodon. I really want to like this Pokemon because it is a bit interesting, but it is just too defensive for my taste. It has a good niche as a support Pokemon, but that just isn't my style.
I like Aerodactyl and the Unova fossils the best. Sinnoh's fossils don't look too good and Hoenn's fossils are a bit too strange in their designs, even though Armaldo is pretty cute when you think about it.
Tyranitar and Aurorus have impressed me though. The first is what Rampardos should have been, or could have been at least, and Aurorus is a non-grass Meganium. In designs, that is! I haven't played with them so I don't know how good they are on a team.
In Black, I have Archen on my team now and hope that it'll be useful. First time using it. Squirtle is adorable but Carracosta is a bit more badass than Blastoise imo.
Probably because she was using the 5th gen sprites, so there weren't any available for Aurorus and Tyrantrum.
Anyways, my favorites are the two Rock/Flying types- Aerodactyl and Archeops. I always preferred Aeodactyl out of the original 3 fossil Pokemon just because I thought it looked the coolest, and I loved how useful Archen/Archeops was for me back on White. I just wish it didn't have that Defeatist ability, as Archeops would've easily been one of my 3 favorite Unovan Pokemon otherwise.
My favorite fossil 'mon is Cradily. It's one of my favorite Rock types and it was the first prehistoric Pokémon that I've used. I like Cradily because it doesn't give off that vicious vibe that all of the other prehistoric Pokémon have. My least favorite is Archeops. I like its concept and I don't mind its design, it's just that I'm unfamiliar with it.
It's a hard choice between Aerodactyl and Carracosta, but I take Aerodactyl because of all the buildup the Fossil Fanatics gave me and the mega evolution. Carracosta in fact looks cool, I actually like his design and he seems so huggable. Aerodactyl on the other hand may be assumed as a rugged ranger, one who is willing to take the risk at the dire times. He also seems rather uncomfortable to ride on, but that's okay. I'm also seeing his new Mega Evolution with wide eyes because of the sudden potential he gets, however the fact that only one mega can be in a team at a battle means that I search for better options to spend that valuable slot.