Remember this is solely based on my opinion of the best HeroMech.
HeroMechs cost MC, which costs real money. Hence deciding upon what HeroMech one should get is very important and difficult. The average HeroMech will cost anywhere between $20 to $50 (on average around $30 actually). For some people, spending this amount of money on 1 "Cash Shop" item is clearly worthless. All they get is a Mech that's little to no different than the others. A 30% CBill bonus isn't much to go by either.
This means, that to decide on a HeroMech, one must first "know the game". You need to know the quirks of each Mech (i.e. the Stalker's crappy Torso twist, the Jagermech's arms etc.). After this you have to know the cost of upgrades and Engines. XL Engines are super expensive and cost about as much as a brand new Mech.
The whole point is to know what you are after.
By that regard, we'll take the Stalker Misery as an example. The Misery feels like it's the perfect Mech for you. You can easily control the arms without Arm Lock (since they only go up and down), it has a whole host of weapon Hardpoints allowing for different loadouts. It's simply versatile. However you've never piloted a Stalker before, so you think that crappy Twist should be bearable. Or the other bonuses outweigh that crappy Twist. You buy it for around $30. You kit it out however you want. Take it to battle only to realise you can't bear the annoyance of that Torso Twist. Now you feel disappointed in buying that HeroMech and you go to the forums to complain and whine (possibly ruin the experience for others).
Before ever spending that amount of money you should know what you are after first. Actually pilot a Mech similar to it (variant).
Another factor to take into account is the armaments as I said before. You want to make the most out of that $30.
So for me, the best HeroMech is a Mech that comes with as much bang for its buck. To me, I better be getting something that feels it's worth that $30 I'm forking out for one thing. The average Mech costs around 5mil CBills; a XL Engine costs around 5mil CBills and the upgrades (Endo Steel and/or Double Heat Sinks) will cost around 2mil, give or take.
That means I'm looking for a HeroMech that is equivalent of 10mil CBills (it also has to be somewhat unique and looks good). The only HeroMech that meets my requirements is the Firebrand. In addition I've already piloted Jagermechs before and I love them.
I've done all my research on said HeroMech, and will be purchasing it soon.
For others though; in conclusion, make sure you know what you are buying. Test out a variant of that Mech to understand the quirks. Make sure you have a clear "definition" or "requirement" of what you are after and want to see in a HeroMech, then make the choice to buy it or not.
I hope you understood that wall of text. I merely typed everything out in my head without planning or organising the ideas into their proper sections.