R.I.P. Sceptile, Gligar, Dragalge, Serperior and Mega Burd :c
Anyway, on to a nomination I'd like to back up:
Mega Steelix (B) -> A-/A | Having seen several matches with this Pokémon, Mega Steelix always pulls its weight in battle. There's quite a few ways to use it effectively: the SDef Curse set can set up on common walls like Registeel and
Cresselia, then sweep entire teams; the SR set is one of the bulkiest hazard setters around and still packs a decent punch thanks to Mega Steelix's 125 Attack; and then there's also an all-out attacker set that becomes massively strong with Sand support (I know Sand isn't the best playstyle, but both Mega Steelix and Stoutland benefit from it, so there's that). 125 Attack may not seem
that good for a Mega, but its high-power STABs, Heavy Slam and Earthquake, make it hit really hard nonetheless, especially with some Curse boosts. Its 75/230/95 bulk is pretty good overall and combined with its Steel/Ground-type, gives it many chances to switch in.
The problems are that Mega Steelix has somewhat of an opportunity cost compared to Mega Abomasnow/Glalie, as well as its abysmal 30 Speed. Its Special Defense could've been somewhat better, but it's good enough to work. Generally, Mega Steelix is a great Pokémon that's definitely worth the Mega slot thanks to its utility, ease in setting up and great synergy with common Pokémon.
Definitely worth A-, if not A.
While typing this, I see Sceptile moved up, giving it less competition for the Mega slot. I now stand by
A.