Meaning not necessarily that of the character you use, just the one you think looks coolest. I was a fan of Ryu's Metsu Shoryuken, but I must say Fei Long's new ultra never gets old.

I absolutely love Dudley's rolling thunder...even though I keep using corkscrew blow because it's just more useful

" I absolutely love Dudley's rolling thunder...even though I keep using corkscrew blow because it's just more useful "

I like T.Hawk's Raging Slash Ultra.  It looks like, it hurts so much... LOL!

Metsu Shoryuken, obviously. Also, I really like Bison's Abortion Stomp Ultra and Hakan's Oil Hold Combination. 

Adon.

Metsu Shoryuken for sure. Graphically looks amazing and painful. Love how it cuts out the sound, everything about it is awesome

Juri's 2nd ultra Kaisen Dankairaku one is pretty bad ass and guys izume drop

Zangief's U2 is awesome.

C Viper 's U2 and Fuerte's U2

Zangiefs U2.

So far it's Cody, it's like a mix between Akuma and Neo from the Matrix. That man is a BEAST.

Fei Long's Ultra II. One inch punch, cmon.

a tie between Makoto's Sheichusen GODNAKZUKI! and Gouken's SHIN...KU...SHORYUUUKEN! I guess I just like the way you can see the shockwave after the punches xD

 rolling thunder it just looks so awesome and it begins with the games greatest phrase "keep it classy"

" Fei Long's Ultra II. One inch punch, cmon. "

Juri's Ultra II.

El Fuerte ultra 2 is the best.

Hakan's second ultra is hilarious.

Metsu Shoryuken: Because the whole world goes silent as you break your opponent's jaw. 
 
Runners up:  
Yoga Shangrila (Slim U2): I can't get enough of Head on Head violence. 
Oil Coaster (Hakan U2): Squirt out your opponent through your ass: Ultimate humiliation! 
Rolling Thunder (Dudley U1): Punch out! Keep it classy.

Favorite would be Vega-sama's (Dictator) Ultra 1.  Brutal as hell.
 
Runners up:
Mike Bison's (Balrog) Ultra 2 despite how effin' weak it is.  You got punched, sucka!
Dan's Ultra 2 for being referential of Art of Fighting.  Just plain silly.
Fei Long's Ultra 1.  You can't stop the Hong Kong missile.

I hate to be such a fan boy of GUY, but his U2 is pretty badass...

Maybe I'm just a little sick of Ryu II (considering I've been using it heavily), but I'm gonna have to go with Sakura II. I just love the buildup and the energy in her voice.

Hard to say.
 
In vSF4, the only one that I found really memorable was Fei's Ultra. 

In Super, there are just so many. most of the new ultras for previous characters are amazing, but special mention goes to Ryu's Metsu Shoryuken, Fei's One Inch Punch, and Dictator's Psycho Shotgun.

Then you have new characters, Ibuki's laser ultra, T.Hawk's Ragdoll Smash, Guy's Izuna Drop, Dudley's Rolling Thunder, Makoto's Castrator U1.
  
They all look SUPERB this time around. It seems like Capcom really wanted you to look and feel like you were really ruining some dude's day with these new Ultra combos.

For me it's the metsu shoryuken and Guy's U2, they're just so badass.

" Hakan's second ultra is hilarious. "

Ryu's ultra 2, easily.

Sigh....
 I was the first one to make a "favorite new ultra" thread when the game came out and its locked for no reason.  Plus, There was probably another thread on it, But before the game cameout.
 
On a side note: Dudley's U1

I like the chime noise in Guy's ultra, just makes it seem so stylish. But on just the looks, T-Hawks ultra 2 is BRUTAL. Out of all the ultras, that one would definitely kill you.

Shin sho.

" I hate to be such a fan boy of GUY, but his U2 is pretty badass... "

dan's numero uno

Even though i think they showed it way to much prior to release i still love Juri's Ultra 2