I'll get back to you on quickness, but here's a list of them all with their reward value. And you could evaluate their quickness there too.
@Johnstone & @Johnstone 2
ediblePoly.com
All my characters
I really like the Freaklympic story arc. Good story, good enemies, great end map....oh, and 20 merits. That last map is a Scrapper paradise.
Hero Merit Reward Listings and Villain Merit Reward Listings via Paragonwiki.
Bookmark it. 
I'm a big fan of Division: Line from Angus McQueen. It's 13 door missions, no defeat alls, no hunts, no deliveries. If you run it for speed, you should be able to do it in under 90 minutes, and it gives 21 merits, plus plenty of Vanguard merits from defeating Rikti.
...I forgot what experience means.
During a drunken respec attempt (yes, I play on Pinnacle) I accidently sold afew hundred million worth of sets and uniques. To rebuild myself to my former glory is going to take many merits as many of the IO sets and Uniques i had are now extremely hard to come by/expensive.
Which Oro flashback arcs are fairly quick, fun and offer a fairly decent amount of merits upon completion. This would be blue side.
Thanks very much in advance.