What to spend rare candy on?

I would put the RC into Mewtwo but not necessarily power up. Entei and Raikou are not that worthy of investment anymore and Moltres has less general usefulness than Mewtwo. But of course all this depends on what you have and what you need. Getting more bag space is also very advisable.

I wouldn't say any are overly worth powering up, all are either very limited use (all the mewtwo) or theres better options coming before they'll be useful (mamoswine destroying ice beam mewtwo as an ice attacker and doesnt need RC) Raikou's not a relevant raid attacker (and theres several electrc types in gen 4 that can compete with it) nor is Entei. Sky attack moltres is very strong, but much like psychic mewtwo its effectiveness if limited largely to machamp raids reliably or vs flamethrower or focus blast mewtwo raids. Only invest RC in a mewtwo unless its got Shadow ball. Most legendaries other than rayquaza lost relevance in gen 4 because of the types that legendaries have going forward.

If your aim's machamp solos, Moltres >> mewtwo, higher DPS, faster TTW, and is a more useful pokemon in general for neutral DPS because of sky attacks ludicrous power.

I'd personally hold onto your RC, I;m sitting on around 300 atm, only investing them in rayquaza.

The sky attack moltres, and maybe a psychic Mewtwo (especially if you want to solo machamp raids).

That said, 79 rare candy isn't much. You're probably better off buying bag storage and saving enough to power a return of Rayquaza, or groudon, or a gen 4 legendary. 4 days of max free coins will get you +50 storage.

79 rare candy is barely enough to bring one legendary to level 30. It shouldn't be a problem to save that amount in your bag. I think you should consider whether your bag space is large enough or you are prioritising items far less relevant (and easy replaceable) than RC.

I know this doesn't answer your question, but if you wanted to be prepared to make a competitive team of relevant pokemon when they are needed, you would need a larger amount of rare candy ready to spend in the next "must have" pokemon.

As stated by others above, your bag storage is a more pressing issue if 79 RC is clogging it up. The storage upgrades are the most beneficial items you can drop coins on.

If you absolutely HAVE to spend it I'd sink it on Mewtwo, even if you don't necessarily power any of them up. If you can get a Shadow Ball one from someone then do that. Otherwise keep it on hold until a legendary comes by that you'll either get immediate use out of or that you like better - I'm holding onto ~600 rare candy that I initially was planning to blow on the Sinnoh legends and that's half my bag space.

As someone who's a spender, not a saver, I would do one of the following:

1) Save it to spend specifically on a high quality Groudon

2) spend it on SA Moltres over your other options.

Continue saving it and expanding your bag.

Tell us their IVs and more about your local metagame.

If I had any I would dump it into Shinx

Probably the Mewtwo.

Then I would consider powering up the best Mewtwo and give it Ice Beam (if it doesn't already have Ice Beam) as Ice Beam is going to have pretty good coverage over the 3 Dragon Legendary Pokémon in Gen 4. Not the BEST coverage, mind you, but Ice will do neutral damage or Super-Effective damage to all of them. Which is nice.

I have nearly 900 rare candy and no legendary I care to spend my stardust on. Dialga will be a no-brainer. But Palkia? Meh. Giratina? Meh. Heatran? Meh. Lake Trio? Meh.

Darkrai, however? Check! And my coveted, eventual, seafoam-green shiny Lucky Latios? Double-check. Until then, I don't even have a species to dump it in (Latios is contingent upon getting someone to trade me a shiny if I don't get one myself).