Everest or Annapurna ?

I have hiked all of the tourisity treks in Nepal, and some untouristy treks as well, but I can't rank them number one, number two, ... For the cultural experience, none of those tow because both are touristy, commercial and money oriented. Otherwise Everest might be the most impressive trek.

I would rank the Everest region higher for "in your face" mountains. Its hard to beat. I'd rate the Annapurna region higher for a cultural experience because you are covering a greater elevation gain and thus more cultural variety. The Annapurna region also has more villages that I find more culturally diverse. I agree with willemspie that both areas are touristy, commercial and money oriented. But, I think you can avoid this a little more on the Annapurna Circuit, especially if you get into more of the numerous side trips. If you do the Annapurna Circuit, certainly take the upper route to Upper Pisang. Spend the night in Ghyaru. Take the side trip to Kagbeni.

If you want more of a cultural experience in the Everest region and you have the time, start your trek at Jiri or Shivalaya. This is the old trail to Everest that climbers took prior to the airport at Lukla. Not many trekkers take it now; they opt to fly into and out of Lukla. The Jiri trek offers a great cultural experience.

Annapurna Circuit used to be the best, before the 23 day trail was shortened to 6 (in the eighties it started from Dhumre 600m and ended in Pokhara, that is where the fame came from, several cultures, peoples and vegetation zones). Not much cultural variety on the standard Lukla-EBC section, but hiking in from Jiri/Shivalay makes up part of that.

If you have to decide between Annapurna and Everest, I would take Everest.

Consider Manaslu Circuit with Tsum Valley. Takes around 25 days and has replaced AC as the best original culture & scenery route in Nepal. The only downside is that it is restricted area and needs at least a guide.