I agree that several of the ZFs appear to have the best "wide open" performance. Alas they are pricey, but you may find that with the introduction of the ZF2s, the slightly older ZFs may now be affordable. It's funny, people who think they need the very, very best performance will throw money at it, so if you just wait a while, you can get some good deals when something incrementally better hits the market
For low light work, I do like RFs a lot, and there are some lenses that are quite legendary. If you find something with a nice big viewfinder, you can have all kinds of fun with that, e.g. the bessa T with a 21mm lens and aux VF is very hard to beat, even though the lens is only f/4. There are faster, longer lenses available though, of course. And it is true that many RF lenses deliver very good performance wide open.
With SLRs, I find RF confirmation (in the viewfinder) very useful, and also scale focusing plus an auxiliary finder can be handy. Unmentionable objects with live view also are fun for this, but on the other hand, their glowing screens quickly make you the most noticeable object in the room. Anyway, I have a Nikon 50/1.2, and that thing is a weapon at wide apertures. And of course, just because it is f/1.2 doesn't mean you have to shoot it at 1.2, but you do get to focus it at 1.2, which of course is a treat.
Indeed most lenses are not at their best wide open, but that is seldom useful to know. Right about now is when some lp/mm wanker pulls out some charts and shows that MTF isn't quite at its peak wide open... What!Ever! We shoot wide open because we like to and/or because we have to and/or we like to use shallow DOF... and all that is fine. No need to justify any of it to somebody who'd rather have an MTF chart in their bag than a 1.2 lens.
Go with your gut. Have fun and don't let technicalism slow you down! Shoot lots of film, and post your shots to the gallery.