Pokémons to power up!

1 & 9. Venusaur is my personal favorite, especially with that moveset and vaporeon popularity. You might as well keep that dragonite maxed, those pulses hit so hard.

Maybe 5. A lot of people have been putting them in gyms lately and stone edge is pretty damn good.

1) If your gyms are anything like mine (filled with Vaporeons and Rhydons) then yes.
2) How are you on Dragonite counters? If you are hurting for them then sure, otherwise no.
3) Nope. Clefable's not good enough on defense.
4) Ehhh. I'd say no. Ghost isn't particularly strong against anything and there are better generalist attackers.
5) Yes.
6) Maybe, but after #5 was done.
7) Yes, unless you already have a better one.
8) Um... I mean I like Flamethrower better than Fireblast for some things... this one is debatable.
9) Yes, if anything can actually keep a gym at the moment that thing will.

1 - Highest DPS moveset although I personally prefer shorter cd charge moves. Sure the grasses got nerfed slightly in the recent update, but they're still good against Vaporeons and also people are putting in Rhydons in gyms now, although you've already got a Vaporeon for Rhydons. It can take down a 2-2.5k Vaporeon as is but if gyms are mostly stacked with Vaporeons and you've not got a better grass Pokemon, it's worth powering further.

2 - Keep it for training purposes. I personally wouldn't power up a Wigglytuff for attacking because I have other Pokemon that would do a faster job. Yes Pound is the highest DPS move in the game and Wigglytuff has STAB, but Pound isn't miles above the rest, like the typical 6 damage 0.5s moves, at least not enough to make up for Wigglytuff's low attack stat. For defence it'll get kicked out after one fight, so it doesn't matter how good the moveset is if it only gets to use it once.

3 - Same as Wigglytuff, but worse for training since Clefable doesn't get STAB on Pound and Dazzling Gleam sucks, whereas Play Rough is basically Psychic with Fairy typing instead. Technically good moveset for defending, but Clefable's max CP relative to other common defenders will probably put it at the bottom of gyms, so it'll get kicked out first and won't have a chance to use it's moves in more than one battle. Personally I don't care for defenders anyways, with gyms being easy to take down, so I wouldn't power it.

4 - No. Low CP means it requires a lot of investment and it still won't be that good of an attacker. On paper, Gengar is a good attacker because of it's high attack stat and Shadow Claw is near the top in terms of quick moves (11.58 is close enough to Pound's 12.96), the problem is the move is often too slow to use it twice in between common enemy quick moves so you lose like a third of your theoretical DPS because you need to dodge due to Gengar's low HP, and Gengar's typing doesn't really give it a niche against a certain Pokemon, whereas something like Pinsir / Scyther / Parasect have one super good matchup that makes them situationally useful, which is Exeggutor, so Gengar's just going to do neutral damage, and there are better Pokemon for neutral DPS.

Ghost moves do more damage to Psychic pokemon, but Psychic moves deal extra damage due to Gengar's poison typing.

Ghost moves also do extra damage to ghost types but the only ghosts in the game right now are Gastly and it's evolutions, and Gengar isn't a good defender for a whole variety of reasons. Gen 2 only introduces Misdreavus as a ghost type, who likely won't be a good defender due to it's low base stats and it's strength in the main series coming from status moves, which aren't in Pokemon GO.

5+6 - Not sure. Personally wouldn't if it's for defense, because I don't care for defenders like I said, and Rhydon isn't a strong defender due to it's typing. For offense I don't know if Rhydon's any good, I haven't got one or a Pokemon that has the same moveset.

I've heard that Rhydon's a good attacker against Snorlax so try it out. Rhydon's got high stats overall and it's rock typing will reduce the damage of Hyper Beam and Body Slam, which are the more desired charge moves on Snorlax. You do take extra damage from Earthquake though.

Don't worry about having Rock Smash or Mud Slap against Snorlax. Their damage values will be the same against Snorlax since STAB multiplier is the same as super effective, and as long as your charge move isn't Megahorn, it's fine.

7 - Worth powering up. Not much to say. It's a Vaporeon, and it's got good IVs (or average if you're picky). Doesn't necessarily need powering up though, attackers are favoured that you can take down Pokemon 2-3x your Pokemon's CP.

8 - Worth powering up if you want something for grass types and don't have Arcanine. Like Vaporeon, doesn't necessarily need powering up though, because it'll still be able to take down a 2500 Exeggutor or whatever grass type in it's current state. Personally I prefer the 50 energy moves with lower cooldowns over 100 energy moves because of dodging, but it's preference. Some players prefer using high damage moves as finishers.

9 - Good defender. I don't care for defenders because of a lack of players in my area (or at least on team instinct), but if you're in an area where gyms do get trained up and can hold for a few days, then it was worth the investment I suppose.

You may want to reference this article (https://pokeassistant.com/stardust) regarding Stardust spending. As the most limiting resource available in the game it makes sense to spend it wisely.

It speaks (and shows a nice graph) about how CP/health gains slow down as the Pokemon gains levels. (eg Leveling from 20 to 30 gives more CP than leveling from 30 to 40) Some points to consider from it: at level 30, the Pokemon is 91% of its CP total, so leveling them higher is less worthwhile than instead leveling another Pokemon also to 30.

Personally, I leveled some Pokemon to 36.5 and have stopped spending the high amounts of Stardust and Candy on them and instead switched to using it on hatched high IV level 20 Pokemon that I always wanted to level up, but instead had felt compelled to keep for top Pokemon.