For merits, I do the following:
1) The Weekly Strike Target TF. Since the double merit reward is once per week PER CHARACTER, I'll run it on multiple characters during the week.
2) Signature Story Arcs. A lot of these can be soloed in under 15 minutes and the 4x merit reward (20 merits) can be claimed once per week PER CHARACTER.
P.S. The bonus rewards on the Signature Arcs are kind of weird. Every character can get a 'free' run of each chapter. So you can pick a character, do all 12 chapters on that character, and earn 240 merits in a single sitting. Then you can pick one chapter, run it again for 20 merits and that counts as your run for the week. Then THAT CHARACTER has to wait 7 days to get the full reward again.
P.P.S. Some chapters are pretty hard. I can solo them all on my scrapper. Some are pretty easy and I can solo them on my empathy defender.
P.P.P.S. If you're going to do the arcs, definitely run through them all once for the story and amazing unique maps. As far as merits are concerned, the final chapter in each arc is much longer and simply not worth it.