I have tried a few of the RDA's and my two favorite's are:
https://www.fasttech.com/products/1363601
and
https://www.fasttech.com/products/1345903
The V5 is great. Used it for a while with no modifications, recently drilled out and added a second air hole for duel coils and it awesome. I use it at around 1 ohm with duel coils. Even with no modifications I liked this one a lot.
I just got the M5 a day or two ago but so far it works well. It is small so not a lot of room to work and with no holes in the posts it is a little tricky to set up. got it working now at about 1.4 ohms and it's nice, probably going to drill out the hole just a bit if I can find a drill bit that is small enough.
The nibus2 and the ones they had for that were made to match the K100's are bad, I would stay away from those. I also have the cyclone clone but it has been sitting in the cabinet waiting to be set up but looks like it could be another winner.
Haven't tried any of their RBA's, I really like my aga-t2 and just want to upgrade to a nicer model of RBA, but I hear good things about the cobra.