There's a few actually, like Furfrou probably being the one that caught me by surprise the most and despite not having used it I'm happy they added that one to the dex.
I have to say that Hawlucha was really a surprise so early on, and it was a tough opponent. Though a great member to the team.
Zoroark for me. White 2 is either my favourite or second favourite Pokémon game and that crafty fox was one of the standouts from that gen.
Zorua, although unfortunately I've deserted my Zoroark because it gets one-shotted far too easily. ;_;
Most definitely Zorua. Such a wonderful addition to my team and Spoiler it made the battle against Ultra Necrozma easier than it could have been.
Out of the USUM Pokémon, I have to say that my favorite of those would have to be Kommo-o, since it was a typing combo that I've been wanting to have for a long time, but overall I would have to say Charizard, because of Charizard.
Tyranitar and Lopunny, because they are both favourites of mine. I also had both on my team in Ultra Moon.
As most others said, Zorua. Even though it's a glass cannon its come in clutch so many times. I'd also like to say Vikavolt because now you can get it a lot earlier than in SM, which is honestly so good.
I caught a Buneary in the heal ball given to me by my character's mom and kept it throughout the game. I was pretty happy to see that the Lopunnite was available at the battle tree, which I believe was event only in the previous game.
I loved Zoura when I heard on Serbii.Net that you could catch it so early at the trainer school I made sure to leave a space for it and I was lucky enough to find it after like five or six other encounters in the school.