Probably the strongest candidate, as a very bulky counter to the Normal-type gym tanks which also destroys the Fairy-types used to wall Machamp. Very future-proof and a good gym defender too. Main drawbacks are its limited usefulness in raids (but the same could be said for my maxed Blissey) and low current level.
What to max out next?
I have only ever maxed out three Machamps, a Blissey and Tyranitar, with all my other attackers and defenders at level 30. For the past few months, I have been hoarding stardust for Gen IV, not powering up even my best FS/SA Moltres, a second 93% SD/SE Tyranitar or my 98% BS/So Alolan Exeggutor.
Incredible luck in the last few weeks has given me four mons that are too good to not max out. Decisions have to be made, as my future Rhyperior, Mamoswine and Electivire army will also need considerable stardust and CTMs (currently have 35). There is also the upcoming defensive rebalance to consider. In Singapore, optimal gym offence and defence is far more important than optimal raid offence.
For each, upvote = yes, max out and downvote = do not max out. Reply to specific post = discussing decision for that specific mon.
Answers
Just did a Gengar raid with a friend who only started playing in August, is just Level 29 and hasn't got many of the good Pokémon. They used 2 Meteor Mash Metagross, level 31 and 33 (we had Windy weather for Beldum day). I used 2 level 38 Groudons and a level 36 Shadow Ball/Psycho Cut Mewtwo.
We tied on Damage. They didn't go through both Metagross, I went through 2 Groudons and got to my Mewtwo, and we still tied on Damage.
Power this one up.
Good gym defender and useful for many legendary dragons (despite being weak to them), but seems over-hyped as a generalist gym attacker (compared to Metagross and Raikou). Faces competition from Garchomp (but I have yet to get a Salamence) and multiple legendaries (which will require RC investment).
Cn/IB is great for Giratina and eventually shiny Rayquaza, until Mamoswine shows up. Cn/Ps is very future-proof but not that useful (I can already solo Machamp with level 30 non-legendaries). PC/Fo is less future-proof as a middle-ground (between Machamp and Metagross) counter to the Normal-type tanks. Will never be able to learn Shadow Ball.