Stardust spending suggestions?

Hey man! i recomend metagross, tyranitar, raiku, SB mewtwo, rayquaza, dragonite, moltres, salamance, chansey (still the most time wasting defender) and venasour in that order.
I do not recomend slacking i have one and it's useless it only scares away lvl 30 and under trainers. I rather put one of the previosly mentioned pokes in the gym. Also groudon and kyogre are the best currently in the game but they still dont have their signature moves available (the strongest moves in the game). Since I doubt you'll be able to TM it to the move in the future I would hold on. Plus nothing wrong with investing in a legendary but there are a couple pseudo-legendaries like metagross, dragonite and tyranitar that deserve to be maxed out.

It depends how you play the game. Are you struggling with raids? Are you looking to get serious in the boulder cup? Do you just want to be able to beat your pals and rivals in PVP?

If you have enough candy, maybe consider getting second charge moves for some of your best pokemon.

Depends on what you already have.

If your focus is on raids, Dark Tyranitar is the best long-term investment.

I'd say save your dust if you dont have good water/ice/grass or electric types right now.

If powering up electric types, i'd recommend Electivire over raikou if you dont have a lot of raikou candy. In my oppinion the rarecandy will be much better spent on legendaries like rayquaza, mewtwo and when we get it, darkrai. Groudon and kyogre are also great options, though groudon a bit less so given ground sees little use in raids, but is a monster in PvP. All of which are supreme in their type in both DPS and TDO, Raikou at present is out DPS'd by the cheaper Eletivire, and when magnazone is released, it will out TDO raikou because of its steel sub typing when against moves raikou doesnt resist, but steel does (blizzard kyogre/water types with ice moves in general, flying types,) Raikou will be a balance of both, however is far more expensive in demanding RC that I feel are better used on other legendaries, given the rather limited effectiveness of electric types.

For gyms, the big three that I feel are the best to invest in are Machamo, Rayquaza and Metagross. Machamp for the blissey, and metagross and Ray for everything else, and to a lesser extent Moltres with sky attack for giving a second move to be an excellent flying raid attacker and the best fire attacker that can deal with gym defense metagross. Machamp and ray will also serve well against Dialga and Palkia raids respectively, and machamp being useful in Darkrai raids as well, if we get them. Metagross is, at least in my oppinion, the best generalist around, fantastic natural bulk + typing, and meteor mash with its stats make it hit so hard while resisting most gym defenders and can blow through all but itself and milotic with relative ease. It also has no threat from Dialga or Jirachi at being outclassed as a steel attacker barring a move rebalance, leaving its only potential threat generation 7 Solgaleo (which is years away)

Tyranitar is also an ivestment for long term raids. Both Double dark and double rock are projected to be the supreme sets of the type for generations, and have match ups through every generation. Double dark tyranitar alone has 4 remaining legendary match ups for gen 4, not including the remaining Deoxys forms or returns of old bosses like mewtwo, lugia, latios/latias, or if niantic feels like adding it, origin forme gratina. Double rock follows similar, though its main match ups are only of legendaries that will make returns, all the gen 1 birds, Entei, Regice.

Whatever is useful for current raids and holds up well in the future is the most worthy of dust. I wouldn't recommend powering up future raid counters in advance, no matter how good pokemon they seem. Niantic could do something unexpected at any moment.

If you really want to spend dust on future raid counters, Machamp, psychic-counters and dragons would be good investments for the rest of gen 4 legendaries.