For me (and I guess for a lot of others) was the Omaha mission. But I really enjoyed the mission right after the Omaha one. It gave me a "Band of Brothers" feel. And I also liked the first mission.
The worst one was the Tank mission. Just didn't do it for me. Slow controls, easy to die and in all not that fun.
Though what pissed me off most was the Sniper level. That just did my head in... But it was a good one when I could actually kill them.