I like Super Saiyan 4, but I don't like how Goku grows his pants and boots back. His fur could easily cover his crotch.
The pants and boots is what completes the design for me. I think they compliment the SSJ4 design really well, unlike Vegeta. SSJ4 Vegeta looks like a walking contradiction, a hairy metrosexual.
Anyway as for the topic, probably standard Super Saiyan. It's the boring answer, but it's too iconic for me to say anything else. SSJ4 is the more interesting design, but I'm not sure I'd say it's the better design. Again, it feels a bit standard because of how accustomed everyone is to it but I think it speaks volume to the simplicity and sleekness of the design that it's become as much of a cultural norm as pepperoni on pizza. Ultra Instinct is my favourite based on utility since it's more than just a simple power-up but again, I just can't go against the classic. I would go against pepperoni though.