I have too many "favourites" to pick just one, so I'm just gonna go down the really long list.
>Samurott
Average stats, left behind by everything else
>Eelektross
Just slightly too low defenses, really low speed, not quite worth the effort to get even though he's a bro
>Umbreon
Dark type is now abysmal, no offenses at all, Sylveon is just better
>Chesnaught
Needs speed and special attack swapped round, lacks a better Fighting STAB than Hammer Arm, weak to so much shit
>Gallade
No mega, Psychic blows, low Defense, average Speed
>Aurorus
Rock/Ice, no special Rock move, low speed
>Klinklang
What movepool?
>Vanilluxe
Ice typing, movepool issues (at least he got Freeze Dry now)
>Typhlosion
No mega, no dual typing, poor movepool, exactly the same base stats as normal Charizard, and forever in obscurity in terms of usability
>Scolipede
Would say average Attack, but he got buffed big time this gen but he kind of lacks bulk
>Yanmega
Bug/Flying type, gained no extra speed during evolution, ass Special Defense
>Cacturne
Slow AND frail, never a good combination
>Golurk
Slow, not quite bulky enough, similarly to Eelektross