Hoo boy, picking any one favorite is hard to do since I tend to appreciate all of the pokemon designs for one reason or another (save for one exception but I digress) but if you exclude any legendaries or starters/popular 'mons, and in no particular order, I'd have to go with:
1.

Love the sleek design and I had a lot of fun using her as a glass cannon in my casual runs!
2.

Favorite Gen 1 Eeveelution and a good bulky water type all around (Ok so maybe
one popular 'mon)
3.

I'm a sucker for cute Pokemon and Alcremie pulls that off flawlessly (and being dessert-themed without making the design look kind of wonky to boot!)
4.

I love reading up on history and Claydol captures the fun mystery of ancient times perfectly, not to mention being really fun to use
5.

/

It's a toss-up between these two as I used both pretty heavily back during ORAS on rain teams, love the designs as well!
6.

This pokemon is near and dear to my heart as the very first pokemon I had to hunt for all the way back during RBY, and I proudly used her in Stadium to beat down the Gym leader castle
7.

Out of the bevy of Gen 1 Normal/Flying Pokemon Dodrio has always been my favorite, fun design and just as much fun of a playstyle too
8.

/

/

Yet another tie, as I love these guys for much the same reason, their designs giving me flashbacks to Godzilla vs Hedorah and just being good bulky Poison-types all round helps!
9.

As a kid who grew up with Leafgreen and was too impatient to search for

I grew to appreciate having a bulkier 'mon to help make up for my admittedly sloppy battling, and that movepool was just great
10.

I have no real explanation for this little guy, save for it's just so
cool