I take it that you wouldn't see some brands in your shop unless you are an authorized repairman for that brand, like Markbass? For the most part, I mean. I find it interesting that you don't see many threads on TB about Markbass trouble. And there was a reply in a thread some time back about the number of repairs (I think it might've been in Europe--but nonetheless it's a yardstick). As I remember, Markbass scored very high on reliability. Of course, anything can fail, though.
Yeah, we're not authorized MB, and no one that I know that has MB gear has had a problem with it. I may get crap for this, but I personally don't like the MB sound. We are authorized Peavey, and I've only seen a couple of Peavey bass amps come in -- a lot of guitar amps though. A lot of Ampeg SVT 4's too - for some reason that amp seems to have a lot of problems (actually, it's not usually the amp's fault, but the user's -- people don't read the manual! With that amp you have to have a special speakon cable to use it in mono bridge mode -- use a regular cable and your amp will fry eventually). Someone mentioned GK -- don't see that stuff too often either and a lot of people like it, too much brightness in the GK sound for me though. I had a Peavey TKO 65 for like 25 years, never had a problem with it and wish I hadn't sold it - great little 1x15 combo that for low watts really packed a punch.
Trying to think what we haven't seen in the shop at all -- never seen an Aguilar come in, had one Glockenklang come in but we couldn't get parts, never had a Genz Benz in, never Ashdown, have had Mesa in, but we usually recommend sending Mesa gear direct to Mesa as they are thorough and cheap and do a bunch of component upgrades when amps come in, plus they are fairly close to us, so shipping/taking it there is not a problem. There's also a regional thing I've noticed -- some amps are more popular in different parts of the country, so we may not see as much of the amps that aren't really popular around here. But we do have a GC, so whatever they sell, we see (we do their repairs for them) - seen at least one of just about everything GC sells (except Markbass). I only do the repair thing part time (I have a real job that actually pays the bills - repairs wouldn't cut it

) so I pretty much specialize in restoration/repair of vintage tube gear and build tube guitar amps as a hobby.
But again, this thread is about best brands and that is totally subjective and up to the listener's ear, but I can speak with at least a tiny bit of authority on reliability based on what I see come in to the shop. AND some of the BEST sounding stuff can be fairly fragile, so reliability and sound quality are not necessarily related. Wouldn't we all love to have a '64 Ampeg B15? (not the new re-issues) but would you take that amp to a gig? Me neither.