As Pyro's said, it really varies from level to level. In my opinion some of the best levels have been:
TFL
Siege of Madrigal (For sheer atmosphere)
Flight from Covenant (Less combat, more stealth)
Shadow of the Mountain (Defensive position)
The Road North (Few units, few combats)
Across the Gjol (Excellent level all round)
SB
Gate of Storms: Some heroes, some massive combat
The Great Library: For the same reason as Shadow of the Mountain
Twice Born: Classic
Others
Nightfall: Aside from my bias from involvement, it really was an awesome level.
A Winter's Night: Great level in general. Lots of combat
Morte Della Luna: Some nice use of 'new' creatures
Coming of the Dark: All of these were well thought out and challenging
Sciron's Solo Campaign: Also had some good levels
There are doubtless countless others, but these are the ones that stick with me even after a year's absence from the game. Capture even half of the concepts in these levels, and you'll have a great campaign.
MOST IMPORTANT for me, is that it have a good story. Well written journal entries, plausible scenarios, and most of all- the black feeling of desperation that TFL captured so well.
Good luck, can't wait to see what you come up with.
LR
