Well, there is no best Pseudo Legend
Honestly in all Pokemon there is no best Pokemon. You can have the best typing, the best stats or the best movepool, but it still could not be the best. That being said here are the pseudo legends and my thoughts on all 8.
Dragonite: It has a great HA (mulitscale), it is pretty well rounded (can be mixed sweeper), it has a pretty good movepool, and access to DD (works well with multiscale).
It doesn't have the best typing (x4 weak to ice, plus a weakness to fairy, dragon and rock). It doesn't have a mega.
Tyranitar: It has a good ability (sand stream), it has a mega, boosting it's stats, and it's a different type than all but one of the other pseudo's.
It has pretty bad typing (x4 weak to fighting plus 6 other weaknesses). It
Salamence: It has a good ability before and after it's mega evolution. It can be a mixed sweeper. It has a good movepool. It has a mega.
It has the same typing as dragonite (not very good typing). It loses moxie (better than aerialite imo)
Metagross: It has a good movepool. The one of the best typing of all the pseudo's. It has a mega. It has access to a priority move (helps a lot).
It still is weak to 4 things. It doesn't have the best abilities of all the pseudo's.
Garchomp: It has pretty good typing. It has a mega. It runs great with tyranitar. It has a decent movepool.
It's abilities aren't that good. It has a x4 weakness to ice.
Hydreigon: It has a good movepool. It has a good ability.
It doesn't have a mega. It's typing is pretty bad. It's ability isn't the best. It can't quite be a sweeper (not enough speed) it can't quite be a wall (not enough defenses).
Goodra: It probably has the best typing of all the pseudo's. It has a good movepool. It can be a great wall, especially with acid armor and it's high sp def.
It's ability is meh. It doesn't have a mega.
So there really isn't a best pseudo legend. They all are very good, but they can all be beaten by different Pokemon. Some of them could beat each other, but then be beaten by a different pseudo.
If you have any questions please ask in comments!