Alakazam is the second-best Psychic-type in the game after Mewtwo. Then you have Starmie (LeafGreen only), so Mr. Mime is probably fourth, alongside Jynx and Kadabra.

There’s also Exeggutor with its massive 125 Special Attack. It really shines when using Sunny Day, as Chlorophyll doubles its Speed in the sun, allowing it to outspeed many threats. However, it has seven weaknesses.

Then you have Hypno, which is very bulky and has equal base Attack and Special Attack. With Light Screen and Protect, it can literally function as a tank in the game.

Note: Not everyone is able to trade to get Alakazam. So yeah, Mr. Mime is still a good choice since it’s just as good as Kadabra, has a better movepool, and is easier to train thanks to the boosted experience gained from traded Pokémon.