Alakazam

I would be more inclinded to use something with a bit more bulk...either higher defence (Steelix etc), or more stamina rather than a glass cannon.
Of course anything that has both def and stamina IS the meta.

Then place Donphan, Steelix, Slowbro, Slowking, Muk, Cloyster, Lapras, Tentacruel, Ampharos, Umbreon, Lanturn, Porygon2, Arcanine, Charizard, Venusaur or Golem.

All better than Alakazam.

Don't listen to people who tell you what you should be putting in gyms. Any mon you choose to put in gyms is a defender. I have a C/P Alakazam, powered up to level 35 (CP 2598) that has collected many hundreds of coins for me, sometimes sitting in gyms for weeks at a time. Is my Alakazam singlehandedly preventing people from taking down the gym? No. Is any other pokemon? No. Putting a 'Zam between two defending Gyaradoses, for example, complicates things for the attacker, and makes him switch out a Jolteon and waste whatever energy he might have built up towards a charge move.

I tested my 'Zam as a defender, by prestiging against it with a few different mons that were very close but just under the CP of the 'Zam, and I tried to pick attackers that matched up favorably. What I found was that the 'Zam was fairly easy to beat, and the battle was over quickly, but he dished out a lot of damage before fainting. So, play your way, and make the game interesting for yourself and others.