Craglorn is where a lot of people hang around. LFG for trials, selling and doing skyreach runs, trading, etc.. Wayrest is probably the best place for duels but you can find people dueling in eldenroot or mournhhold as well. And vulkhel guard in auridon is good for duels too, a lot of lower level duels specifically (but not only), plus a lot of people seem to hang around there.

I usually spend my time in wayrest and craglorn. Pretty much all the time im not in cyrodil or doing something specific in another zone. Its nothing like it was back in one tamriel where you could find every capitol city packed, as well as craglorn, with people dueling, testing builds, trading, selling/buying, and LFG for dalies.

** Plus whatever zone has a current event is usually very populated for the duration of the event. For example, vvardenfell was pretty packed for the morrowind event over the last few weeks. I think the thieves guild/dark brotherhood events are coming up soon.