Sorry, but this kind of assumptions make no sense, for one thing the teams working on each emulation driver are different it's not like it's the same guy juggling with 10 different drives at the same time and they don't focus on all systems at the same time, like now for example they are mostly working on the PSX driver. PCE emulation on Mednafen is very mature, besides being cycle accurate and has nothing to envy from Magic Engine, it's also free and is constantly updated (not really the PCE driver since it doesn't need it), when was the last time you saw a Magic Engine update?
Oh and PC-FX emulation on Mednafen beats the hell out of the paid Magic Engine FX just saying.
I wouldn't say Mednafen is the best NES emulator though, there's still quite some work to do there, same for PSX, but if they keep their standards I wouldn't be surprised it would become the best emulator for these systems later down the road.