→ C-
I honestly don't see what niche this mon has that
actually makes it worth using. With the mega slot being so valuable right now I don't quite see why you'd run this over regular Garchomp. It's faster, about as strong (with life orb), and still breaks a lot of Pokemon, except for Stall, but quite frankly, there's better stallbreakers out there that do not waste your mega slot.
Man I really am on Mega Garchomp Jury duty arent I
1) I dont know why you think mega slot is all of a sudden so valuable. It's true that mega lati@s have gotten more viable in the metagame recently, but
that's really it. Every other mega has basically stayed the same (for all the flaws you state mega venu having, it has really enjoyed the increase in clef/gastro usage). The mega slot is only slightly more valuable in balance/BO builds that can support mega lati@s and that aint all of em.
2) Because a mega slot is pretty expendable, why wouldn't you run a mega where you can? That's the only reason people run shit like mega latios over LO latios (if the mega slot was so valuable, we wouldn't see mega latios everywhere (i get that mega latias is a bit different)).
LO chomp is terribly outclassed by other better wallbreakers like SD lando-t, and honestly, its outclassed by mega garchomp too, because mega has more bulk and power, whereas the 102 speed tier just gets less and less impressive anyway. You can even run jolly and keep it un-mega if opponent is running smth like volc/tapu lele/mega medicham to keep your semi-useful speed tier and mega later when they're dead/you need the bulk more. Mega chomp is a good check to heatran and that's one of the few reasons to even use chomp nowadays, and LO chomp really can't do that half as consistently as mega can.
I see that you note there's better stallbreakers that do not waste your mega slot, but there is where I see the irony, because almost all of the notable stallbreakers in this tier use a Z-move, which is WAAAY more valuable than the mega slot. SD kartana, SD lando-t, CM/taunt tapu lele, NP hoopa, SD tapu bulu, Kyurem-Black, Heatran, Greninnja, and this is ignoring the weirder ones like CM Keldeo, double dance Mew, Taunt NM tapu koko, TG manaphy, etc etc.
There's no fucking way I could list out all of the unique and amazing wallbreakers that value the Z-move, and that's just the breakers. We haven't even hit mons like z-hurricane torn-t, sg magearna, flyinium z gyara/dnite/mence, etc etc.
By using up the comparatively less valuable mega slot to get a solid wallbreaker that
A) breaks well given the right support
B) is very bulky (more natural bulk than ferro) and can check hard to cover mons like heatran
Of all the z-move wallbreakers i listed, none of them except for keld and manaphy can really switch into tran, and the megas in the tier that can do it (latios, zam, gyara, ttar) require their own support too. And as far as Mega Garchomps support goes, after trying to build and test with it I found that cb ttar+mega garchomp actually form their own little wallbreaking core quite well. Cb ttar can pursuit any stray z-hurricane torns or scarf locked hp ice lando-t looking to revenge mchomp, and chomp in turn uses sand to pop things like unaware quag, every clef, pressure lando-t, celesteela, tapu fini etc so cb ttar doesn't have to spend too much time guessing the right move and coming in multiple times to break sufficiently (also relying on stone edge lul).
Obviously that's pretty specific, but thats why Mchomp is C and mega latios is A. My point is that it is a completely viable choice on par with pokemon like mega camerupt, alolan muk, thundy-t, nidoking, and mega gallade.
So TL;DR Mchomp is a totally viable wallbreaker that saves you the z-move (way more important than the mega slot it uses), solidly checks heatran, and pairs well with cb ttar.
Keep it ranked at C
i swear i'll get replays over the weekend or smth :]