A/UX is certainly the most interesting for the Q800, but 7.1 is very, very fast on a 33MHz 68040. The overhead in the later systems will slow even a Quadra down a great deal, while admittedly adding functionality. I personally like the simplicity of 7.1.
Much depends on what you want to run on it and put into it. I'd say 8.1 mainly if you want a second large(>4GB) HFS+ formatted drive or drives on a Jackhammer card or the like; 7.6.1 if you want Cyberdog, or to connect to a X.4 share (with some updates - there's a good guide to this on system7today.com), or to do much internet-wise; 7.1 if you just want it to be fast, or if you'd like to set it up as an Appleshare 4 File/ Print Server; A/UX 3.0 (better than 3.1) if you want to tinker with a UNIX/MacOS 7.0 hybrid.