Well, lots of people come in and start threads here and at TWC about which is the best mod...

Firstly, there are awards/polls on both sites which - though they may ignore some mods - do generally mention and award good ones ;)

Secondly, how on earth can we know what sort of mod you like in order to answer your question? Historical, fantasy, both? Time period? Enhancement? Focus on gameplay? Focus on graphics? etc....