Well seeing that it's sort of an opinionated question, there technically isn't one. But if you want the best Psychic type all around, I'll give it to you!
So after looking at like 60 Psychic type pages, I see now that Unown is the best Pokemon all around. enjoy! :D
Or at least that's what I WOULD be saying if it were still yesterday..
So a fun fact is that Mega Gardevoir has the most BST out of all non legendary Psychic types.
So the best Psychic type would be Alakazam, Gardevoir, and Metagross. these guys are great. They have pretty good base stats (and Mega makes Gardevoir pretty cool) They also have some nice movepools. Gardevoir (in my opinion) is the best out of them all with it's Mega.
If you use Mega Gardevoir you can wreck lives if you give it Giga Impact or Round. Once you get that TM for Round teach it to a Pokemon (almost all Pokemon learn it) and then teach it to Gardevoir! Use it with the other Pokemon in your party and Round will do precious damage. But I'm assuming this is for competetive play, in which Gardevoir's physical attack power isn't very great compared to it's special attack power, so Giga Impact certainly won't do as mush Round! And that's without the Fairy and Psychic type moves, Pixilate just changes the Normal moves into Fairy..
I think your best bet (if you aren't battling competetive) would be Mega Gardevoir. If you are battling competetive then it's still a pretty good choice. But I gave you the three Pokemon which are (probably) the best non legendary Psychic types around for that.