I want to power up one pokémon from level 20 to 30

The Cloyster. Excellent moveset, will be a dragon slayer.

Cloyster

Not the Alakazam. They don't make good defenders due to low stamina. There are better Pokemon that can get Confusion + Psychic for defending e.g. Slowbro, Exeggutor, heck, even Golduck.

As an attacker you can make up for low stamina by dodging.

Personally, I'd evolve Cloyster because if I had a Scyther or other bug type Pokemon for Exeggutors, I'd rather it have X-Scissor instead, but Bug Buzz works fine too.

Alakazam has low stamina so it's not good for defending. In Scyther's case, you should save him instead of powering him up because in second gen he gets an evolution (Scizor). So that leaves you with Cloyster to power up.

I won't power up these 3 since Gyarados can fill the gap on taking down Psychic like Exeggutor and Slowbro and it is a good match up on vs high tier defender even with Lapras.

Cloyster first. Scyther second if you don't want to hold out for Scizor

Im a big fan of evolving level 28-30 wild caught pokemons. I think all three of these would be perfect candidates for high CP no stardust invested evolutions. Theres so much to build and none of these are gonna be high utility so I'd recommend going for cheap evolution's and saving your stardust for gen2.
But if you must Cloyster is the only one. I guess technically any master trainer should have both a FB/B and FB/IB level 30 ICE mon.

I'd say Alakazam, but for attacking and not defensive purposes, and maybe the Cloyster too if you need a good dragon slayer.