Rare Candies

It depends, of you have a good Mew2, give them to him, Rayquaza is also a good investing for generalist purposes and for future dragon type Raid
Bosses, also Groudon is worth investing because of the coming Regis raids

I'm trying to decide that for myself as well. Maxed Raikou, Entei, and Articuno. L37 Mewtwo, which doesn't really benefit from any more levels. L30 Groudon and Kyogre, which aren't really needed right now. I have a 98% Ray, but don't want to power it up until its catch period is over (on the chance I find a 100%--but that's never happened with any legendary, so I'm not expecting it). And not much on the near-future horizon that I want.

Doesn't hurt to wait, unless you are super strapped for space.

I'm also thinking of a similar question, already powered-up
Rayquaza(100), Kyogre(100), Groudon(98), Raikou(98) and Moltres(98) to lvl36-37,
Lugia, Articuno, Zapdos, Entei to lvl30-32
I don't have a 82+ Ho-oh or Suicune (couldn't and didn't raid them)
It looks like Regi trio and Lati duo doesn't excite me either.

Probably going to power-up Entei to 35 for fire fang breakpoint, then start wasting on luxury power-ups for weather trio

Mewtwo is out of question, there is no ex-raid or sponsored gym in my vicinity.

Tangela.

Imo:
1. mewtwo.
2. If your not set for fire in the future raids (regice, celebi, registeel, jirachi) go for moltres and/or entei.

1. Mewtwo
2. Groudon (more useful in the upcoming meta than Kyogre)
3. Kyogre
4. Raikou
5. Rayquaza (meta-relevant, but small difference to best non-legendary alternative)

After those, I'd put rare candy into Moltres or Entei as they will be top tier against 2/3 of the Regis so quite meta relevant.