7.0x (including OpenDOS on which Enhanced DR-DOS was based) were the real ones I think. Most 7.0x versions were similar, and I think 7.03 was the last one with a complete package of utilities.

I don't know if anyone had actually bought a DR-DOS 8.0 instance. From what I remember that version was officially sold at ~$200 for 5 users.

DR-DOS 8.1 existed for a while until it was pulled due to controversies. 8.0 (which was said to be unaffected) was also pulled and 7.03 became "latest" again.

Caldera DR-DOS 7.03 was pretty much the last full version for desktop use - later versions were targeted at embedded/industrial use and lacked many things. You can get a (fully functional and 100% identical to the real full version) trial version of Caldera DR-DOS/OpenDOS 7.0x (plus Enhanced DR-DOS) from here: http://www.zx.net.nz/netware/drdos/caldera.shtml (despite what some think, Caldera DR-DOS is not freeware and it is not open-source in any way - its effectively shareware, you're allowed to try it out for a while after which you're required to buy a license somehow)

Novell DOS 7.0 with the latest patches installed (available here: http://www.zx.net.nz/netware/drdos/7.shtml ) should also be fine. Its lacking some enhancements Caldera made in in DR-DOS but probably nothing you'd really miss/notice (Wikipedia has the details on what 7.0x adds and what some versions of 7.0x lack compared to Novell DOS). Here is the back of the Novell DOS 7 box which may give some ideas about what changed from DR-DOS 6:

I don't have a copy of DR-DOS 6 but the impression I get is there isn't much reason to prefer it to Novell DOS 7. But if you wanted to run it, the final patches should be here: http://www.zx.net.nz/netware/drdos/6.shtml