For 24/7 use (No OC) in very hot ambient temperatures, which one is most reliable, workstation, OC or gaming motherboards ?
This is another great question, and something we actually get asked quite a lot. If you're using your CPU without overclocking it, and especially for 24/7 use - I would recommend a workstation motherboard.
But, this doesn't mean the OC and gaming motherboards won't be of equal quality, and would somehow cause stability issues with 24/7 use. If you asked me this 10 years ago, I would've not recommended a "gaming" or "OC" motherboard - and always workstation - purely based on the gaming motherboards being the OC motherboards back then, and they weren't the most stable.
Things have changed so much over the years, with OC and gaming motherboards becoming incredibly great, with huge stability and feature sets that make them a blend of workstation and gaming motherboards - without the workstation price tag.
At the end of the day, for ultimate stability for 24/7 use - I would recommend a workstation motherboard from the likes of ASUS. Their WS family of boards are rock freakin' solid, and built with all of the features under the sun.