Whoa....it's a hard question to answer, not helped by me being away from the game for some time now. it can vary depending on long-term vs short-term pleasure. (Of course, if a short-term play-through is intended, that's not so important.) So many favorites are geared to my second play through play style. For instance, "The Elder Council". My avatar's existence centers around its road-patrol assignments. It's his post-Crisis life's work. His nine-to-five job, so to speak. It therefore goes to the very top of the list. Was he not committed to patrol duty, TEC would simply be one of numerous fine quest mods.
Likewise with another quest mod, "Kragenir's Death Quest". It provides a zillion and one things to do. Most are mundane, often humorous. If played right, it can take many months finish, assuming you do. My avatar still hasn't completed some quests. (He could have, were my play-style different.) It has to be near the top of the list, again due to longevity. There are certainly a number of 'flashier' shorter-term quest mods out there.
The second mod my 2nd play-through centers around is "Companion Villa" (duh!). Like TEC, almost no play session doesn't involve Villa.
I'm going to put on my thinking-cap, study my load order, and try to whittle it down to 10, if that's at all possible.
--------------------
|