So I started with Blissey + Alomomola because it's a fantastic core that can wall by itself a huge portion of the metagame. I opted for a cleric set on Blissey without wish, because though it's always nice to have protect to scout for pursuit, I don't like running Blissey without Toxic and I believe Blissey to be the most effective Heal Bell user in the tier. I opted for a max defense set with rocky helmet on Alomomola to punish U-Turn and to weaken problematic threats like Heracross and Sharpedo, so you can't afford to stay in on Primarina or Hydreigon and Toxic but it's a stall team anyway I have a lot of defensive support in the back and I really want my Alomomola to take physical hits as much as possible.
Once I found my main defensive core I decided to add a defog core, because I think hazard control is one of the most important trait of stall and needs to be thouroughly thought. Mega-Altaria seemed really good as it's able to switch on threats like Superpower Hydreigon Crawdaunt and Infernape which are usually a big problem for stall teams as well as benefiting from Natural Cure which allows it to defog against stealth rock setters with Toxic or Scald. And here is my favorite move on this team : Haze on Altaria. With Haze, Mega-Altaria is able to deal with Suicune, Infernape, Reuniclus, Slowbro-Mega and help against Celebi and NP Lucario (if you don't mega). Next I wanted a mon that could defog against what forces Altaria out, which is why I originally chose Gligar, to defog against Cobalion, Mega Aggron, Mega Aerodactyl but I ended up changing it for Scizor which can defog against Mamoswine but can't stay versus Cobalion and Mega Aggron. I made this change because Bullet Punch is really great for revenge killing annoying threats like Terrakion or Mega Altaria that could break through Quagsire, and with its steel type it's immune to toxic without taking being crippled by Scald. I opted for Toxic over another attacking move to ease the duel against Hippowdon and Swampert and I just felt that U-Turn wasn't necessary.
I started looking at the threats to my team and automatically went for Quagsire to help dealing against the various physical setup sweepers in the UU tier. Then, looked at what felt and noticed that I had no flying resist and no stealth rock users. After thinking for a while, I thought about Nihilego, which fits perfectly the team as it provides Stealth Rock support as well as being a fantastic counter to Togekiss, Work Up Mega Pidgeot and Chandelure, which are usually really threatening to stall. It can also switch on Gengar and Hydreigon if needed.
And here it is, it's not the most original team on this thread but I really believe it to be the most solid and that's why I presented it. Regarding the threats, Magneton could be a problem but I don't really on Scizor too much to deal with certain threats, the only problem would be Haxorus + Magneton but it's pretty hard to build around that. Outside of Magneton, Nasty Plot Lucario forces Mega Altaria to stay in regular form and can win the 1v1 with good rolls, and Celebi also wins the 1v1 versus Altaria, but with Scizor and Blissey behing which can both toxic it should be okay.