Best use of rare candies?

I hear ya, I wish MewTwo would arrive so I can unload a bunch.

But in my opinion, rare candies should only be used for legendaries, 20km for 1 candy is too much. you can get 4 Larvitar, Dratini, etc candies for that and nearly 7 Zam or 20 Geodude candies in 20km.

Use it Only for 20 kms.
I walk something like togepi for gen 4.

Completely agree with the comment above - I would only use them on legendaries.

I had a similar amount of rare candy and just 'topped up' each legendary bird to 66 candies which will get one of each to level 30 when I finally decide to power them up... freed up a little bag space.

Chansey and legendaries for me. Everything above the 300 goes into Chansey as long as Mewtwo and Raikou haven't arrived.

Basically use them for whatever you want. If you want to power up legendaries, that is the most logical decision. If you want another Tyranitar or Dragonite, then use them on those. It would be less "optimal strategy", but if that's what you want to do, then go for it. Especially when you have so many rare candies. I've used mine for the legendary birds mostly, but I've also used some for Tyranitar and Dragonite (about 30 each) because I got impatient! The best game strategy? No. Made me happy? Yes.

Legendaries even if you can't power one up you might later, or Tyranitar if you don't have enough of them, which is what I'm doing even with Tyranitar as a buddy, since I know I'll power up more of them eventually.

I've used enough to power up a Lugia to 30 and just put some in Raikou. Since I only have 2 Legendaries that are over 90% (2 Lugias), I'm almost tempted to power up both Lugias just for Focus Blast Mewtwo, but that's probably overkill since you either have the numbers to beat or you don't and Mewtwo without Focus Blast is not that hard. You also nerf your DPS if the raid group was big enough with Lugia...

I agree they shouldn't be used for anything other than Legendary Pokemon because even if there are not ideal ones now, future gens will have new ones with probably new moves, better stats, etc...

I'd even advise against Ttar since they hatch from eggs and their buddy distance is only 5km. You can also catch them in the wild (select areas of course), but they spawn in my area and I've pinaped all of them.

Since I'm an IV snob, I had near 550 Ttar candies before I evolved/powered up my 1st Ttar...(had 7 hatches, some during double candies...). It's TONS easier to get ttar or any 5km walking candies which can hatch from eggs, esp during a double candy event. After powering up 2 Ttars now, I have near 400 candies and may have to evolve another if I don't get a good IV one from a raid...

TL:DR - Use it on any Legendary pokemon that you eventually plan to power up. You will continue to get more rare candies so hoarding tons is unnecessary, esp if you have no space...Get more space too. Spend on Mewtwo after you catch one...

All depends on what you want to. Mine have gone to Tyranitar because it's hard to find a group around here for them, and I can't get any eggs to drop larvitars.

Besides, it's a premier attacker in the current game, hence the usefulness. Beyond that, I'd say legendaries probably.

Or anything which eludes you, such as mareeps or such.

Not conventional wisdom given the focus on Legendaries, but personally, I'll be using a bunch towards my first metagross when gen 3 drops, assuming it isn't a raid boss.