My least favourite would have to be Metagross, It's not that I don't like it's just that it's my least favourite design compared to the others.
It is also my least favourite as I like Dragon-Type Pokémon more or at least Dragon/Lizard looking Pokémon in Tyranitar's case anyway.
While I do like all of them, I'll have to go with the one I find myself using the least and that's Metagross. Metagross is a good Pokemon it's just that it's types are two that I don't really care for and I'm not a huge fan of its design either.
I absolutely despise Hydreigon. The design is weird, and it strikes me as odd that almost all of the Pseudos are dragon type, anyway. Other than that I find that it sucks as a Pseudo. It's got power, sure, but it's a little too easy to bring down, and the fact that it takes so long to get to the final evolution is absurd. Overall Hydreigon is easily my least favorite Pseudo.
To be quite honest, I always forget that Garchomp is one of the pseudo-Legendaries, so I guess that would have to be my least favorite. I don't think that Salamence is all that great either (all in all, they're very similar to one another), but it looks a lot more like a dragon than the hammerhead land shark that is Garchomp.
I like all of them, not sure I have a "least favorite" among them. The one I never used was Metagross, though. 3rd gen being my least favorite gen and having zero postgame meant I never bothered with Steven's gift Beldum.
Omega Ruby & Alpha Sapphire, the day Pokémon pulled a Dallas and jumped the shark.
Salamence just looks really really fugly. Tyranitar is an absolute pain to train and get, but the final critter is really worth it in the end. Salamence, on the other hand, is just a fugly Dragonite. No special abilities or anything. Just a boring-ass Dragonite. Snore.