I feel that this thread is pursued by two distinct groups of people, both looking for something else..
One group is looking for nic base with as less flavor and bite as possible, in order to prevent interference with the flavors that are mixed into it, or just because they like to vape unflavored base (where unflavored means unflavored).
The other group is looking for nic base that has a pleasant flavor that comes from the tobacco it was extracted from, in order to support the tobacco flavors in their mixes, or just because they like to vape unflavored base (where unflavored means without additional flavorings).
This means that it makes no sense to talk about nic base in terms of good or bad, without telling what you mean by that.
I am in the second group. I look for nic base that has a pleasant flavor. This means that I am not very interested in trying NN or VT (apart from the fact that they do not ship to Europe). From the ones that were discussed in this thread I tried MFS and RTS, and from these to I prefer the flavor of MFS (I have a liter in my freezer). From the European suppliers I used to love EcoPure Regular and AtmosLab Mist, but I slowly started to dislike taste of the former, and dropped the latter because of inconsistent batches. Tried a few others which I didn't like much, but didn't try Inawera (allegedly too pure for my taste).
Several of the bases I've tried have an nasty taste of what I consider (not sure) to be traces of some kind of chemical solvent that is used in the extraction process. Whatever it is, it tastes bad.
RTS and MFS don't have that and therefore I consider them to be free of undesired substances that were not part of the tobacco it was made from.
(btw in my case it's all 100% VG)
I would add at least one more group; those that have very limited funds and are looking for something that serves their purposes while costing as little as possible, but do want to avoid anything truly nasty.