U1 is the best of the 3 in my opinion, and probably the best low level AD&D module - it has never failed to entertain. U2 is very nicely written, but could be over far too quickly, and U3 is very ambitious and incredibly difficult if the party are not careful.
I like N1 a lot too.
Over the last year I've run several parties though a variety of classic modules, with mixed success. I've used U1 twice, both times were a blast. T1 has had a lot of staying power as one party chose to rebuild the Moathouse and use it as a base. I tried B2 with the same group, and found out they aren't really that enthused by humanoid-filled sandboxes! We've had fun with it, but despite my best efforts to get them interacting with the factions, they don't seem to care all that much. N1 started very brightly with another group, but sadly I'm only the backup DM for that group and only run sessions about once every couple of months, so they lost the thread of the plot a little. We also ran UK5, and though the players had fun, I found it a bit bland.
Based on my own enjoyment, practical use, and player feedback, U1 wins the battle of the low level modules hands down!