There are a few very wide lenses for M's I haven't used, but the ones under discussion I have.
I have and have had the 18/4 for about 8 years, and it's really quite good. However, since I've had the WATE since it came out, I don't know why I keep the Zeiss. It has no real advantage over the WATE.
I don't have a 16mm Hologon, but I've shot with it. I have had the 15mm Hologon for Leica M since the early 70's when it came out, and while I enjoyed shooting with it for a long time on film, when the first 15/4.5 VC lens came out, it didn't see much use anymore, and neither would the 16mm. They're just not much use on digital (both will fit and can be used) as there is extreme falloff; much greater than on film, the corners are poor and the colour shifting is significant.
The new 15 VC is much better on digital than the original, but of course bigger.
Of the 21's, as someone suggested, get the SEM and be done if top optical quality is needed on digital. On film, a case can be made for the 21/4.5 Zeiss, but it really doesn't work well on digital with soft corners and colour shifting.
Other good 21's are the 21/1.8 VC and the Summilux. The latter is of course quite expensive and I probably wouldn't have bought that had the VC been out. Kobayashi had been talking for years about a fast 21, and I had pretty much given up hope by the time the Summilux came out.
In any case, I wouldn't consider any Hologon for shooting right now with other, more modern options available, and especially since it doesn't appear likely that your 15 or 16 will receive much use. A 15 VC is a much better choice. If 18 is suitable, the Zeiss is a good, not terribly expensive choice.
BTW, in the late 60's and early 70's, before I gathered enough money for a Super Angulon f/3.4 and the Hologon, and after I had given up on the SA f/4, I used some Canon lenses adapted for wide stuff, including first the 19/3.5 and then the 17/4 and 21/2.8 FD lenses. I was quite happy with them as improvements over the SA f/4.