Got 3 candidates I am not sure if to ultra evolve them yet. I don’t think they are worth it, but I want opinions of people who ultra evolved them and can judge it better.
Prismarix
Bovolcus
Dragaia
Let’s get started here 
Bov can be a perfect bait in a poison team when evolved so if you have good poison monsters then go for him, if not last biter as itachi says
I already got bovolcus on my second account and on my main I got it recently. I am just thinking on saving team costs since now bovolcus has 8 points as dragaia and priamaryx and I need to know if they are worth the 5 more points they cost me if I use them as evolved legendaries I can easily evolve all of them no need to decide who first or so. It’s about doing it or not doing it at all.
Bobolcus only gain poison revenge, not worth 5 extra cost. Dragaia depends if you need retreat and the added stat (for penguini I ultra him because I use him for assisted thunder).
I don’t have Prismarix, don’t know what his moves are.
Don’t evolve bolvicus, it only improves his pvp ability which is mediocre anyway and it ruins his potential in UC etc because without it you can use him alongside sleep stuff.
Dragaia is worth evolving just for retreat tbh, it’s suprisingly useful.
Don’t know what prismaryx gets for his last move so I can’t help you there I’m afrad.
prismaryx
Ok @ bovolcus that was my first thought. Unfortunately I already did that on the second account but well it ain’t that catastrophic 
main account matters more.
Prismaryx last move is friend hp share and that’s the question, does not seem worth it but I don’t have match experience with it in the ultra form only 5 star fully trained.
Dragaia gets retreat but I don’t have assisted earth, so wouldn’t know where else in my team to place it. I only see a value in last bite when playing against AI. No good player in PVP would actually let him get to the point where he can use last bite.