I don't have to have any. I learned in my first few patch days of hell not to get too dependent on anyone's mod. However, my vote for the best mod in terms of simplicity, play value, and overall support is hands down...
Clique
It vastly improves the user experience for anyone having to cast. I can do without all others, and technically can do without Clique, but I'd be a very miserable and disgruntled player if Clique packed it in and moved along to the Great Mods of the Past Graveyard.
Once again, great job Cladhaire!
One other mod I absolutely love, but not to the degree of Clique...
Bayi's Extended Questlog
It's yet another mod that really improves the user experience and is relatively simple to use and configure.