love for Gengar?

Congratulations for having a nice collection of yours!

For your reference, three kinds of mons are worth powering up:
1. Attacker with good move set, to get things done quicker (save a considerable amount of time in the long run);
2. Defender with high CP or true defending strength (for topping and holding gyms);
3. Favourite Pokemon. An Eevee lover would max out his Eevee just because he loves it.

For Gengar:
1. Not an elite attacker, even with Shadow Claw + Sludge Bomb. A worse generalist attacker than Alakazam and Dragonite. His Poison typing makes him a bad counter against Psychic types like Exeggutor. Also too fragile even without 1.25 weakness.
2. Not a defender. Alakazam takes him down in 12 seconds.
3. The only reason to power him up.

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Added:
It's almost the same difficulty to get a good Alakazam and a good Gengar. I definitely recommend Alakazam.
Both require good dodging, but Alakazam will truly amaze you when dealing with Fighting types and Poison types.
Alakazam even takes out Chansey (which has become quite a headache these days) a little bit faster than Dragonite.

Genger is a great attacker. It's worth powering it up for sure. A ghost type quick move is great against exeggutor and slowbro but it's as fragile as Alakazam. You have to constantly Dodge. Idk about your stardust stoke but if you feel like it, power it up till 30lvl.

I personally really like gengar and use him when i can, but he has weakness to the ones he should be strong against, the Psychic types. So unless you don't need to worry about stardust, i woudn't power him up more than lvl 30.