my favorite transformations in dragonball series are

Ascended Super Saiyan
False Super Saiyan
Great Ape
Super Saiyan
Super Saiyan 4
Super Saiyan 5
Super Saiyan 6
Super Saiyan 7
Super Saiyan 8
Super Saiyan 9
Super Saiyan 10
Ultra Super Saiyan
Unlock Potential

Super Saiyan God Super Saiyan.

On top of being an insane power boost, and leaving the Super Saiyan with blue hair... it is just really cool to be able to say Super Saiyan twice in the name of the power up.

I thought about it for a bit before trying to put thoughts into a post, and part of me wants to say ultra instinct but I realize that's recently bias so I'll go with the original Super Saiyan form.

Legendary Super Saiyan. Massive bulk, unlimited ki reserves, endless stamina, great resilience... They ruined it for me in Dragon Ball Super, aside from removing all its epicness from the first movie, though Bin Shimada got better at voicing Broly as the years went by.

Also, I'll take the golden haired version and not the greenish tint. You know, just so those skinny golden haired jumpsuits will be crushed by my heavily muscled open torso to demonstrate who truly is the Super Saiyan of legend beyond kings and princes.

Oh, and I'm also a big fan of Super Saiyan 4. Many people dislike it, but I find it quite logical: the pinnacle of a Saiyan's power is to master the power of a Great Ape in a humanoid form. And the red fur is because golden fur was just too gaudy and they looked for something (slightly) more discreet.

Thumbs up to SSJ4 Gogeta and his scarlet hair predating Super Saiyan God. It's funny how the strongest character of the Dragon Team for way more than a decade was succeeded by another red haired transformation.

my favorite is the super saiyan kefia with fusion with the 2 girls

.:.:::....,, you’ll know why

Super saiyanjin 2 is the go to for me. Watching gohan become this arrogant show off fighter and inflates his ego while crushing cell’s is something awe inspiring

My favorite transformation is Ultra Instinct. Even though it isn't as easy to obtain as other forms, it is incredibly powerful and it also looks godly. I think part of the reason it looks so cool to me is that it isn't as drastic of a change as other transformations in terms of his appearance. His hair becomes silver/white and doesn't stick up and the aura around him is also a cool silver/blue. Very cool all around.

I would have to say Coolers final form, looks like Shredder from TMNT the dang thing. Plus stuck in my mind with Drowning Pools Told You So playing in the background as Goku got his shape carved into a mountain.

Super Saiyan 2-4 are prime. I can't really think about it and choose one (just kidding I can, SSJ3) but those three are really fantastic forms. Goku is in his prime when he's serious/angry and typically those forms have been the best examples of him being like that.

Although I have been enjoying Blue lately. At first it was just a lame SSJ recolor, but once the Black arc started they really set off to making it the "ultimate super saiyan form" rather than just another step on the ladder. Every time there's a cinematic for Goku and Vegeta transforming into Blue, it's always hype. (see: Broly movie)