Nothing running on the SmartThings platform is reliable: they can and do push out updates and make platform changes without notification they can break anything, even a Z wave light switch. That’s just where things are at right now. 
http://thingsthataresmart.wiki/index.php?title=Bug:_First_Reports
As far as individual brand/models that are well engineered and would be reliable on a more reliable platform, there are quite a few good choices.
As @sidjohn1 said, the primary issue is to make sure you have a good network backbone with enough repeaters of the right protocol if you are using mesh. But mesh isn’t the only option. 
Best of the best switches
In my opinion as both an engineer and a customer the Lutron switches are clearly the quickest and most reliable. They hold a lot of patents, including the original smart dimmer patents, and it’s hard to beat them for home lighting, particularly switches. But they don’t do outlets. And you will have to buy their “smartbridge“ to get smart things integration. (one bridge can support up to about 40 devices.)
Other Brand Notes
As far as Z wave devices, if you look at them as good/better/best, then the GE/Honeywell are at the high end of the “good“ budget category. However, Other than the cheap Chinese imports, the GE are probably the least reliable over time. But they’re very inexpensive, So it’s worth it to a lot of people.
Leviton are at the low end of the “best” category and are very reliable, but do tend to be noticeably more expensive. Also notice, though, that they have a five year warranty, more than double that of almost all the other brands. That in itself is a good indicator of reliability. 
At this point I would probably put both Zooz and Inovelli In the “better” category, although they both have new model lines coming out this year which may boost them even higher. Both have innovative designs, but are also trying to keep the prices low. Since both are newer companies, it’s difficult to judge the reliability after, say, four years, but they are certainly at least as good as the GE but with more features.
Linear/gocontrol tend to be very reliable, and fall into the “better” category just because the materials quality and feature selection isn’t quite as good as Leviton’s.
What are your measurable requirements?
But let’s back up a minute. A lot of people who say they are looking for the “most reliable” will end up buying the least reliable, that is the GE, just because of the price.
Because I myself am a Quad And have an engineering background, I will pay more for a truly reliable switch. But I have a quantifiable definition of reliability.
I look at MFOP (“maintenance free operating Period”). My minimum target for appliances and smart home devices is an MFOP of six months, and preferably a year. That means I don’t have to touch it (literally!) after installation. This is also sometimes called “set and forget.”
Unfortunately, at the present time there’s no way to get this with SmartThings. Any individual device might have an MFOP of 12 months, but there will be a smartthings glitch of some kind at least once a month, and usually about every two weeks.
Still, there are certainly differences between brands and models.
If you really truly mean “most reliable,“ as in “longest MFOP,” then I would go with all Leviton zwave plus outlets and all Lutron light switches. Or use Leviton zwave plus for everything if you prefer that option.
If instead you meant “most reliable, but I don’t want to spend more than $35 per switch,“ then that’s a different conversation.