Who to next add to my attacker squad?

Don’t stop at Level 30, Max them ALL out.

I think you should not prioritize option 1, 4 and 5 due to their ATK IVs. Having lower ATK IVs require additional dusts and candies to achieve same performance of 14/15 ATK IVs.

I prefer powering up option 2 and 3 because they have 14 and 15 ATK IVs, respectively. In addition, Raikou (option 2) is one of the best counters for flying and water type pokemons.

Definitely take the raikou to 30, he is very useful in gyms and now in Ho-Oh raids.

You can't go wrong powering up the C/FS Espeon.

The Raikou is definitely worth powering up, but you can make it a long term project. It has a lot of uses, though.

Alakazam with ShB is also useful against Gengar, although I've found my powered-up PC/SB 'Zam to be less effective than I'd hoped against both Gengar and 'Zam bosses. With those IV's, though, I'd definitely power it up sooner or later. If you don't like SB, you can TM it, and you'll get either FS (very useful), or FB, which is great against Blissey and Snorlax in gyms.

Tyranitar is not the best attacker in the game, and you are overestimating its effectiveness against all the defenders you named (although it is definitely good against Gengar raid boss, but you want B/C for that purpose). Still, T-tar is a strong attacker that can slot into a number of roles, and you want to have one. But I would put it on the back burner for now, at least until you have more larvitar candies. Don't spend that many of your rare candies on T-tar.

No hurry on the Entei. For Scyther, you want rock types (i.e. Golem), and for Blissey, Machamp is way better, and even the fighting Poliwrath will outperform Entei.

Summary: the Espeon is a no-brainer, and the Raikou is probably the next most useful. The 'Zam needs to be powered up as well, and then try out ShB, and see if you like it. If not, you can't go wrong charge-TM'ing it. Wait a while on the other two. You will regret spending that many of your rare candies if you get a Mewtwo anytime soon.

Do NOT max out or power up too many Vaporeons and Golems.

Vaporeon and Golem's position in the meta are the most tenuous.
There are other water and rock Pokemon with much better attack, but limited by moveset.

These are the potential competitors for Vaporeon and Golem in Gen 1/2, which can usurp their roles if given a better moveset. Also, potential Gen 3 competitors.
The number is the Pokemon's base attack stat.

Potential competitors for Vaporeon (205):
Sharpedo (243)
Kingler (240)
Gyarados (237)
Starmie (210)
Feraligatr (205)

Potential competitors for Golem (211):

Tyranitar (251)
Rhydon (223)
Armaldo (222)
Aerodactyl (221)
Kabutops (220)
Omastar (207)

Many of these competitors are glass cannons, but in a meta where DPS is king, it doesn't matter that much.

Raikou is probably the safest counter, though, if youre really stingy with your resources, you should wait until a week after Ho-Oh leaves to see if Raikou is re-released and try for a better one. 87% 14 Attack best moveset is tough to beat though.

Not sure where my first comment went, hope I don't create a double post here.

2. Raikou is a clear nobrainer winner. It's extremely useful right now for Ho-Oh and will stay a top electric attacker for multiple generations. Solid investment with immediate use = power up now.

1. Powering this Espeon up to 30 might not have any impact on its performance. It hits breakpoints against Machamp and Gengar at L30.5 and L31 but those have only a small effect. I have personally never noticed any difference between L28 and L30 C/FS Espeons in many successful Machamp solos. 19k dust isn't much though so you might as well power it to 30 if you like it that way.

3. As stated already, SB is also good against Gengar. That said, anything Alakazam does in solo raids, any random L28+ Espeon from a wild high level Eevee with whatever IVs does almost as well. I would keep it as a project you can work on later since the IVs are great, but it's not worth spending your fairly limited resources on at the moment.

4. Machamp and Dragonite are way better Blissey counters than Tyranitar and you have one of each already. C/SoB Eggy and FS/OH Flareon are also faster than TTar. I would keep walking so as no to spend rare candy and also to give yourself a chance to hatch one with even better IVs. The upside of TTar is that it doesn't require many TMs since there's only one useless charge move.

5. Flareons do Scyther solos and kill Blisseys just fine without you needing to invest significant resources into an Entei. I would not power it up at this moment. For Scyther solo, a team of your Golem, Dragonite, 4 Flareons should complete the raid easily.

EDIT: Remember that assuming best movesets, Flareon is the best nonlegendary fire pokemon. Flareon is better than Arcanine with any moveset except E/FB, while Charizard FS/OH ties with Flareon's second best FS/FT moveset.

Given the current state of raids, you need more than one of something. Want to take down a Raikou, you should have 2-3 strong Rhydons, not one. And no one has enough Tyranitars unless they have 6 with B/C and 6 with B/SE, as well as a few with Fire Blast just in case the move becomes a useful legacy.

Don't power up Entei until you're all done with the raids - you could always get a better one, unless you have a perfect. Then pick out the best one as a candidate for powering up.