I'd give Galvantula a choice specs and give it Thunder and Bug Buzz.
I'd give Mega Aggron EVs in special defense and attack and give him an Assault Vest to boost his special defense further. I would also give it Heavy Slam, seriously, Mega Aggron weighs 870 pounds, it outweighs the bulk of Pokemon by a rather large margin. There are not many Pokemon who even come close to this weight.
bulbapedia.bulbagarden. net/wiki/List_of_Pok%C3%A9mon_by_weight#List_of_Pok.C3.A9mon_by_weight
I would personally not use Hitmontop. He doesn't have much going for him. I think think of about a dozen fighting types that would work out better.
Gengar looks OK.
I'd give Goodra maybe a Focus Sash or something to recover hp myself. You have it set up as a special wall.
Personally, I would give it counter, rest, ice beam, and Rain Dance. why? No one will see it coming. The opponent will use a physical attacker Vs goodra, which is why you use Counter Unless you get hit by a very strong physical ice attack, you can't really lose. Also, if I remember right, you won't lost a turn from using Rest while it's raining if you have Hydration, since it heals status ailments every turn and the biggest threat will be someone using Thunder on it. Goodra's a special wall that resists thunder.
1. Galvantula already has Bug Buzz and Thunder. Its greatest tool is Sticky Web and giving it Choice Specs reduces the number of opportunities it can use it, then attacking afterward by limiting Galvantula to one move while it's on the field, hence why Life Orb (to give it a near-equal amount of power) and Focus Sash (to ensure Galvantula gets Sticky Web up) are preferred.
2. Aggron is only good in its Mega form. If you're using its regular form, you're doing it wrong, as it has common weaknesses to Water, Fighting and Ground (the latter two are especially painful as they're 4x weaknesses). The two sets I posted take advantage of Mega Aggron's defensive capabilities the best. Besides, there are better Assault Vest users out there.
3. Special walling is one of the things Goodra does best. Being a bulky Special tank is the other thing. Ice Beam is redundant with Goodra's Dragon STAB and if you have it as your only attacking move, you're doing it wrong. Goodra is also too bulky to utilize Focus Sash; also, Focus Sash is a one-time item. Counter is also a gimmick, unless it's used by Wobbuffet. I highly doubt that HydraRest is effective this gen because of the rain nerf. If Goodra is to be using Rain-Rest, it must be paired with Politoed (and you need support to keep it alive) and this team doesn't have it (and wouldn't be a good fit on this team anyway).