Well, best 'Mech overall is generally hard to define. Question is, do you value a 'Mech's versatility? Being able to fill a considerable number of roles and fill them well does not mean you'll get the most damage or the most kills in a given match - or that you'll be able to best every other 'Mech in a one on one. It does mean that that one 'Mech can do reasonably well in all game modes and on all maps if build right, though.
Taking versatility into account, I, like many others so far, have to say Timber Wolf. It can be build for super long range sniping, for long range Gauss vomiting, medium range laser vomiting, short range brawling - hell, you could turn it into a LRM boat if you wanted to. Essentially, if I had to pick one single 'Mech to play the game with, I'd pick the Timber Wolf because I'd have a reasonably good shot at performing well everywhere.
However, if one was to go for the 'Mechs that excel in a specific situation, things become a lot more difficult, in my opinion. It now almost entirely depends on outside factors. The Black Knight has been brought up before and I most certainly think it is an absolute monster once you have to deal with it within its preferred ~400 meter range without any obstacles to mess with its low hard points. On the other hand, given real close quarters combat and an environment with lots and lots of cover, you might want to consider the Oxide for its brawling capabilities and its ability to employ hit and run tactics (and survive, thanks to its structure quirks) to grind an opponent down. And if it's about sniping, well, you could go with an ERLL build on the Blackjack, using its high hardpoints and agility to poke all day, every day.
My personal opinion on this is that versatility should most certainly be taken into account when looking for the best
overall 'Mech. For most other cases, you can think up a set of outside factors that negate a given 'Mechs capabilities to a certain extend - going back to the Black Knight example, it's not going to do too well if it has to navigate a maze of chest-high obstacles that a Timber Wolf can easily clear with, say, the A-LT omni pod.
That's at least my stance on the matter
Edited by Luminis, 02 March 2016 - 06:24 AM.