1: Super Saiyan.
Goku becoming the Super Saiyan is a top 3 moment in the series for me. That whole fight is just amazing. He really made the form feel like a big thing at the time. I honestly can't think of any form to top this for Goku specifically.
2: Super Saiyan God.
I can't really explain why it's so high up, I just love this transformation. From it's design to how it works. The fight with Beerus was really fun and I cannot explain how hyped I was when I seen this for the first time.
3: Super Saiyan 4.
Yeah, the one everybody talks about. It has a great design, it's strong, definitely has hype surrounding it, and is one of the highlights of GT for me.
4: Ultra Instinct (Both Mastered and Omen)
Goku unlocking UI was and still is the best scene in Super. I do not care what anyone says. The form also looks badass, so it has that going for it too.
5: Oozaru.
Monke.
6: Super Saiyan 3.
I probably like this form about as much as Oozaru tbh. Nothing about it screams "peak Goku" for me like some of the others do. I am also not the biggest fan of its design.
7: Super Saiyan 2.
Now if I were talking about Gohan this would be #1 but, not for Goku. He barely touches the form.
8: Super Saiyan Blue.
I am not the biggest fan. At least SSJ2 looks better.
I am not including Kaio-Ken because it's more of a technique than a form but if I did it would be somewhere between SSJ4 and Oozaru.