I've never done a "pure" MR...it has to be a destination I'm at least semi-interested in to justify the cost in terms of my time. But since I can find something interesting almost anywhere, that's not a high bar.

My favorites:
- Multiple ORD-LHR cheap winter weekends when there were crazy route bonuses on pretty much any paid fare. Also no copays to upgrade...I'd usually fly one direction in J and still come out ahead on net RDMs with the bonuses. (I was Plat at the time.)

- Multiple $90-120 R/T fares from ORD to various points on the West Coast. Usually on TW metal routing through STL, so these were kind of painful for a regular 3-day weekend. But the fare was right and there were always promos, so it was still worth it...even if it was just 2 nights in SF, LA, or SD. One of the promos even included Aviators Club access....right during the AA merger period so I could us AA clubs as well.

- Daytrips MCI-DAL when 3 R/T's triggered a 25k bonus. OK, this is the closest I've done to a pure MR...lunch with a friend, maybe a Rangers game, fly home... (AA eventually gave up on this route. Now only WN flies it.)