Charizard isn't #1 for me as he used to be, but I still have to mention him because he was firmly my #1 until very recently. I think it helps that the above artwork from the Pokemon Red cartridge was the first Pokemon I ever saw, which is still my favorite artwork of him in general. "Here, have a fire dragon" was a pretty effective pitch for me.
I always thought Arcanine looked cool, but what really put them into absolute favorite territory for me was finding out their stated height of 6 feet. I always assumed they were the size of an ordinary large dog, but seeing that they were more like this:
I wish Arcanines were real.
I wish I had more to say about Gliscor, but it's pretty simple. It's a scorpion-bat. It looks amazing. The crazy yellow eyes tie everything together. Apparently in the anime Ash had a Gliscor that kept its Gligar mannerisms, which is adorable.