The Mega ST (not the Mega STe) is the best quality machine of the series. Mechanical cherry keyswitches on the keyboard. Easy access in the case to expand (though the Mega STe/TT had standardized VMEbus and the Mega ST was its own custom thing).
The Mega STe had a funkier case, VMEbus, and upgraded specs, but mushy rubber dome keyboard, more brittle plastics.
I like to collect the Mega as the best of the bunch, personally.