Single fans are usually smaller, built for less space cases like itx. Consequently, they also have smaller heatsinks and as such very rarely have OC clock/memory speeds.
The most expensive versions use better binned chips for the vrms/vram so not only can handle the factory OC, but also stand a better chance of higher user OC. Also have the largest and most capable cooling setup.
The middling gpus are all roughly the same, different OC's sometimes but may or may not have the better chips, since they only get tested upto the need. So may or may not do well with a user OC very far above any factory OC. Biggest differences are in cooler design, shroud design, rgb or lack of etc.
There is no 'best' as such, as that relies on perspective. The best for me is a middling as it's the better balance of value, cooling and I'm not all that interested anymore in pushing the OC for minimal gpu (if any) gains.
If a middling can hit 150fps and a top line hit 160fps, that's going to make 0 difference on my 60Hz monitors, making any OC whatsoever a moot point. So why pay more for something I can't see or use, that's going to run equitable temps since the card isn't pushed to its limits.
The Mechs are the value line, the Gaming are the upgrade. M series has 1130MHz, and the MX series has a 1280MHz base speed, which is slower than the speed of their predecessors, the Gaming, and the Gaming X series. They both respectively have 1185MHz and 1280MHz base speed. Talking about the boost clock speed, Gaming M and Gaming MX have 1560MHz and 1620MHz, respectively.
So Mech, Mech OC, Gaming M, Gaming MX, Gaming, Gaming X is the order. Pricing isn't accurate as it can depend what's on sale, or discounted as different versions have different release dates.