This is such a subjective question that stating you have a definite, exact answer would be counter productive.

Sure, the current meta game is dominated by Infernal Axe mechs. But I have an energy mech that can annihilate those in a heartbeat. Does this make my energy mech to best? Of course not, as it's extremely weak to physicals.

My point is that every mech will have a weakness, regardless of how perfect it is originally thought of as.

Sadly though, the mech with the fewest weaknesses is hands down, the Infernal Axe heater. This is pretty sad considering all the other possible mech builds.

Just goes to show how unfair some items are.