In Black 2 and White 2, both fossils are postgame exclusives, meaning neither are a good choice for an in-game playthrough. The best Rock type is probably Terrakion because of its raw power, despite how late it comes. The best Water type is probably Samurott (or Azumarill if you didn't pick Oshawott), and the best Flying type is probably the Braviary on Route 4 (or Sigilyph if you're not playing White 2).
However, if you're somehow getting a Tirtouga or Archen in your game via trading or external means, I think Archeops is more useful throughout the game. This is for a few reasons:
- 140 Attack and 110 Speed
- Ground immunity
- Moves like Rock Slide, Acrobatics, Crunch, U-turn, and Fly
- Can OHKO most neutral matchups thanks to strong moves and high attacking stats
While Carracosta has some positives, such as more bulk and famously not having Defeatist, it's generally worse than Archeops because it's slower and can't OHKO neutral targets without Shell Smash. Archeops is both faster and immediately stronger, meaning it's unlikely Defeatist comes into play very often.
Archeops
Ability: Defeatist
- Rock Slide (TM in Mistralton Cave)
- Acrobatics (Level 28)
- Crunch (Level 35)
- Fly (HM)
Rock Slide is Archeops' most consistent STAB move and will allow it to pull its weight against Skyla. Itemless Acrobatics is the best Flying move Archeops gets and should be the main move of choice against neutral targets. Crunch is one of the only other accessible physical moves and allows Archeops to act as a check to Shauntal and Caitlin. Fly is mainly for overworld utility because nothing else Archeops gets is more efficient, unless you want to use U-turn at Level 45 for some reason.