Another of these thread eh?

Unlike some, I think that's just fine. Often see a few mods that I am not familiar with, that I think might be worth a look.
But as said; the short answer is that there is no "must-have" mods, it is all a matter of personal opinion.
But some come close. Namely those that 'fix' shortfalls in the game, such as:
http://www.tesnexus.com/downloads/file.php?id=10400
http://www.tesnexus.com/downloads/file.php?id=23208
http://www.tesnexus.com/downloads/file.php?id=22368 (for that bashed patch, and the best mod installation method)
http://www.tesnexus.com/downloads/file.php?id=27710
http://www.tesnexus.com/downloads/file.php?id=11280
And some noteworthy mods in my own humble opinion:
http://www.tesnexus.com/downloads/file.php?id=21104
http://www.tesnexus.com/downloads/file.php?id=18305 (Beautiful varied weather, not to mention the interior effects and "Real lights")
http://www.tesnexus.com/downloads/file.php?id=35099
http://www.tesnexus.com/downloads/file.php?id=19628
http://www.gamesas.com/index.php?/topic/1158727-the-unique-landscapes-project/ (all or a few, take your pick)
From
TheNiceOnehttp://www.tesnexus.com/downloads/file.php?id=26389
http://www.tesnexus.com/downloads/file.php?id=31855
http://www.tesnexus.com/downloads/file.php?id=35969
http://www.tesnexus.com/downloads/file.php?id=25078
http://www.tesnexus.com/downloads/file.php?id=34735
From
Arthmoor (author of the latest Unofficial patches as well)
http://www.tesnexus.com/downloads/file.php?id=20053
http://www.tesnexus.com/downloads/file.php?id=35265 (neat simple overhaul.)
http://www.gamesas.com/index.php?/topic/1107706-relz-arthmoors-villages/ (actually a set of village mods.)
http://www.tesnexus.com/downloads/file.php?id=21999
http://www.tesnexus.com/downloads/file.php?id=20218
http://www.tesnexus.com/downloads/file.php?id=16360 or http://www.tesnexus.com/downloads/file.php?id=33003
http://www.tesnexus.com/downloads/file.php?id=31773