I have a few, most of them for aesthetic reasons, because I started and really got into Pokemon when I was younger, so didn't really play it properly. Here's my top 6 (most/all from diamond: I didn't play the earlier ones):
1. Luxray. Luxray looks amazing and was my second ever Pokemon. I love Electric and Ice types aswell.
2. Infernape. My first ever Pokemon, I traded him through games and whole systems until Pokemon Black. Sadly, he is now gone. RIP Blbo.
3./4. Abomasnow/Mamoswine. As I said, I love ice types, especially these huge, heavy ones. I trained a Swinub all the way up to Mamoswine a little while back, so I've grown fond of them.
5. Staraptor. I kept Staraptor with me at all times. It was a great type/s (Fighting Flying I think) and was my flying Pokemon so I could get to wherever I wanted whenever I wanted.
6. Dialga. I know it's a bit nooby having a legendary on my list, but I loved my Dialga. Even though he's legendary he was by far not my best. It's just his attacks were so cool. Like spacial rend. I loved Dialga.