Hi Pomacentridae,
First, there is the option to take a very good standard zoom (ideally with internal zoom and focus - I am not shure such a lens exists) behind planport and use together with wet lenses for macro and WA, but I do not have experience with this, so cannot say much...
I only can tell what is useful/not useful for me personally, from the lens/port combinations I own:
#1.: WA-rectilinear.: Indeed the Pana 7-14 behind Zen DP170 does not give good IQ, especially at 7mm (I almost never use this combination any more). Maybe the Pana 8-19 behind Nauticam 180 gives better results, both because the Nauticam port is a larger section of the same sphaere, compared to Zen and also because 8mm is not as wide as 7mm. There are reports in this forum that real good IQ with rectilinear WA in this range can only be achieved with larger domes, the optimum beeing the Zen DP200 (that is, unfortunately, not sold any more; there is still sold an identical/similar (?) port by AOI, but I did not read in any forum that someone was testing it so far).
#2.: WA-fisheye.: IQ is excellent, already with the Zuiko 8mm (likely identical with pana 8mm), but as Chris says, the Canon 8-15mm (with Metabones 1x Smart adapter) is much more versatile as it allows to zoom in to some extend, what is often very useful. I use it both with Nauticam 140 and Zen DP170, in both cases IQ is brilliant. Eventually it could be used also with Zen DP100, but I do not have this very small dome, so cannot say. Also the Tokina 10-17mm fisheye (with Metabones 0.71x speedbooster) gives practically the same IQ with both domes to my eyes (However, what you save with the cheaper Tokina lens (that is a lousy lens over the water) you spend at least partially for the extra price of the speedbooster, compared to the glassless 1x adapter). To my personal opinion and preferences a fisheye is highly desireable for WA with MFT, a zoom fisheye even more...
#3.: "Normal" range: Like Chris, I am excited about the Zuiko 12-40mm Pro, behind a domeport (Zen DP170, in my case). Due to the domeport, the IQ is brilliant at 12mm (As good as the fisheyes above) and you can zoom in as far as to 40mm. This lens is the optimum for fish portrait, but also for shyer animals e.g. schooling hammerhead sharks (not the feeded ones!).
In case you go for a domeport, I would buy the N120 versions and use with N85-N120 adapter: You need different extensions anyhow for the different lenses, even when using a port with built in extension. The N120 with appropriate extension/adapter costs more in the beginning, compared to a N85 port with built in extension, but when you, at some time, grade up to another camera system (APS-C or FF) you can continue to use the N120 domeports. You have to check the portcharts to see what extensions you need...
In case you go only for a fisheye, the Nauticam 140 is the optimum, since it is reasonably small, eventually the DP100 will work also. If you want to use also the 12-40mm Pro (and/or the 7-14mm), I would go, at least at the beginning, with the Zen DP170 N120: With the N85-N120 34.8mm adapter you can use the Zuiko 8mm as it is. Add a 35mm extension and you can use the Canon 8-15mm (1x) and with a 25mm extension you can use the12-40mm Pro and the Pana 7-14mm. The Nauticam 180 likely works a little bit better with the 7-14mm (still not the optimum), but you have to check out the extensions by yourself...
Wolfgang