old player, new game

Check all your Halloween Gengars to see if they have shadowclaw.

Goodness can be defined in a number of ways. Gym attacker? Gym defender? Raid attacker, which itself is defined by the current raid bosses?

Here is my opinion on clear Gen 1 winners for any of these reasons:

Alakazam
Machamp with dynamic punch (truly indispensable)
Gengar (particularly if it has shadow claw)
Exeggutor (some disagree)
Chansey
Gyarados (particularly if you TM to waterfall)
Snorlax (particularly if body slam)
Dragonite (particularly if dragon claw)

Some moves are indicated as particularly useful from a legacy standpoint. Otherwise, you can change from whatever you had to something better by raiding and getting TM. Have Steel/Rock Machamp? Change it to fight/fight.

Look at the individual Pokémon pages and ask for input if you aren’t sure about whether you need/want to change moves.

Yep, several are! Since you haven't played in awhile, I won't include any of the Gen 1 legendaries that aren't currently available.

--Attackers--
• Machamp - the top fighting-type Mon in the game with Counter/Dynamic Punch. One of the most essential members to any lineup for attacking gyms.
• Golem - the top rock-type Mon in the game with Rock Throw/Stone Edge. This could change if Tyranitar learns the move Smack Down (rock move that got added to the code recently with the same stats as Rock Throw). Not super meta relevant right now, since all the legendary birds aren't current tier 5 raid bosses.
• Gengar - the top ghost-type Mon in the game with Shadow Claw/Shadow Ball. Sadly, Shadow Claw is a legacy move for Gengar, so newly evolved ones can't learn it and the move it was replaced with (Hex) is nowhere near as good.
• Dragonite - 2nd best dragon-type Mon with Dragon Tail/Outrage. Bested only by Rayquaza.
• Gyarados - 2nd best water-type Mon with Waterfall/Hydro Pump. Bested only by Kyogre.
• Exeggutor - Places in the top 3 for grass types with Bullet Seed/Solar Beam, bested by Groudon with Solar Beam and Venusaur with Frenzy Plant (unique move only available during the March Community Day).
• Charizard - will be one of the top 3 fire-type (competing with Moltres and Entei) Mons with the new Blast Burn move that it will be able to learn during the Community Day event this Saturday, the 19th. Only one evolved during the event will learn the move.
• Jynx - Top DPS ice-type Mon with Frost Breath/Avalanche, though very frail. Will likely be dethroned by Mamoswine in Gen 4, and already has some competition for it's niche role as an ice specialist from Articuno.
• Alakazam - 2nd highst DPS psychic-type Mon with Psycho Cut or Confusion/Future Sight. Bested by Mewtwo and has similar performance to Espeon. Deals heavy damage, but is very frail.
• Rhydon - 2nd best ground-type Mon with Mud Slap/Earthquake. Bested only by Groudon. Ground-types aren't that terrific right now due to limited uses/mediocre moves, but that could change down the line, and Rhydon has elite stats and another evolution (Rhyperior) coming in gen 4.
• Zapdos - 2nd best electric-type Mon with Charge Beam/Thunderbolt. Bested only by Raikou. Currently available as the weekly reward for earning 7 Field Research stamps.
• Mewtwo - Still available as an EX raid boss, and is one of the top generalist/psychic attackers in the game with elite stats and a diverse move pool.

--Defenders--
• Chansey - with the new gym system, Chansey has become a top tier defender. It's low CP means it's motivation decays slowly, while it's massive HP and good counter coverage (Zen Headbutt/Dazzling Gleam) cause attackers to waste more time and potions going up against it.
• Snorlax - Another strong gym defender with good bulk and counter coverage with Zen Headbutt/Heavy Slam. If you have a Snorlax with Body Slam (another legacy move), it's also supremely obnoxious to fight against.

Also, when in doubt, the three gen 1 Eeveelutions (Vaporeon, Flareon, and Jolteon) are all good Mons that will do in a pinch for a fair number of encounters.

I'm probably missing a few, but I hope this helps!

Eevee evolutions are still fine.
Gengars( especially with Shadow Claw)
Machamp is still a king in his role.
Gyarados have now fast water attack and is great water attacker.
Alakazam is great Psychic- glass cannon.
Golem is still top rock attacker.
Jynx is top Ice DPS.
Snorlax is good defender, check out yours to find Body Slam on any of them.
Lapras is ok as defender and Ice Attacker. Also check for legacy moves( Ice Shard, Ice Beam).
Dragonite is very useful whole time.
Chansey is good defender.
Exeggutor is good as Grass and psychic attacker. If you have legacy Confusion, also good defender.
Pinsir is good bug attacker.

Check out Omastar, maybe you have legacy moves like Rock Throw and Rock Slide.

Do you have a Omnastar with rock throw? If so match it with a rock charge move if it dosent have one.