Chas, I can't say that I've truly seen any great looking releases of M&MN and even among the ones that I have seen the episode quality can vary dramatically. I'm still waiting for my "Best of TV Detectives" from Mill Creek and that has something like 16 or 17 episodes included so we'll have to wait and see about those. If I were to give you any sort of recommendation (and it is a feeble one), go with Alpha . . . . . the quality is certainly passable (with the above caveats) and, to the best of my knowledge, they have relesed the largest number of episodes of the series to date (eight 4-episode volumes) with Critics Choice close behind (seven 4-episode volumes). I'll try to revisit this after I've had a chance to see the TV detectives box set.
I have a couple of the Alpha releases and just one of the CC releases. They're different episodes on each of the discs, so I can't make a direct episode-to-episode comparison. But, on an overall basis, I didn't see much of a difference among them. Unless M&MN ever gets an official release (I'm not holding my breath), these are probably as good as it gets. One M&MN release that I would tell you to stay away from, should you ever come across it, is the Canadian PD label Cascadia Entertainment's version; it's pretty much unwatchable.
I seem to also recall the Alpha and Critics' Choice volumes being on a fairly even level, quality-wise. I was purchasing both for a while, but ultimately veered towards Alpha, as they appeared a tad more committed to releasing further additions. I'm really quite fond of the series. It's such a breezy concoction. I would love an 'official' release, but I have no idea who owns the master prints, much less if they even exist. I think it was produced by the same outfit that did "Topper," and at least those received a new lease on life in syndication in the 1980s, with nice (although perhaps edited?) prints. Oh, well. It would be the longest of long-shots. And, by the way, was Barbara Britton a peach or what? Wow!
Just a bump to note that four volumes of "Mr. & Mrs. North" are among the CC titles in DD's current $3 sale. Free U.S. shipping and no tax (for me) too!