Playing through X, I definitely tried to purely only use new Pokemon (except I just HAD to use Charizard... love it hehe) so I didn't really fall in love with anything older. Although, I've always LOVED Butterfree, and the fact they gave it a slight special attack base increase made me extremely happy and love it even more. (same with Wigglytuff and Clefable, actually)
As for the new Pokemon, I really didn't like Noivern before the game. I thought it looked really awkward and even kind of ugly. Then in X&Y, I ran into a Noibat, and it was sssSSOOOOO cute. I couldn't resist. So I caught it and named it Cupcake. Somehow she ended up on my team and when she evolved into Noivern, I immediately fell in love. She was a beast! And Noivern's flying animation is awesome and so solid. So the game definitely made me love Noivern a lot. <3
Same with Greninja, only with Protean though. I started with Fennekin but I got a Protean Frogadier in Wonder Trade so I put it on my team and it was so much fun! Protean is one of my favorite abilities now.
Other Gen 6 Pokemon I didn't like so much until I used them: Furfrou, Carbink, and Meowstick. <3 <3