Definitely SSJ1 since that form managed to remain relevant throughout the tail end of the Cell arc and continued to maintain a presence unlike every other form.
Design wise its SSJ2 CG Gohan and manga Future Trunks. SSJ4 is unique but looks too much like a Gargoyles character who's a mutated half-monkey statue reanimated, SSJ3's overly long hair and lack of brows makes it look tryhard and God and most of the Blue and UI variations are not even designs, just lazy traces with differently colored layers.
Ultra Ego had the potential to be groundbreaking if they said Blue Evolution was basically applying SSJ2's principles for powering up into Blue, and Vegeta accessing SSJ3 and reaching the pinnacle of God Power made his eyebrows disappear whilst unlocking Hakaishin capabilities, instead it is a retread with shorter hair.