Those who became Super Saiyan 1

Goku: After Krillin is killed by Frieza. (DBZ: Frieza Saga)
Gohan: While training in the Room of Spirit and Time with Goku (DBZ: Early Cell Saga).
Vegeta: Prior to the 19/20 android battle while training in outer space (DBZ: Android Saga).
Future Trunks: In the alternate future, 17 and 18 killed Son Gohan causing him to become SSJ (DBZ: Trunks Special)
Chibi-Trunks: Not determined... (DBZ: Majin Buu Saga)
Goten: While training with Chi-Chi when he was 6 years old (DBZ: Majin Buu Saga)
Gotenks: After fusing as SSJs in the Majin Buu Saga (Majin Buu...).
Brolly: SSJ1: He was born a SSJ (Movie 8).

Ultra Super Saiyan
Achieved between Levels 1 and 2. USS is more powerful then level 1, but alot slower. I guess they would be from thier muscles growing very large
The only people to go USSJ were Future Trunks, Vegeta, and Goku in the Cell Saga

Super Saiyan Level 2
Level 2 is one step higher then leve 1. Changes in Level 2's include the following
*Green eye color
*Hair is spiked more at the top
*Become even more menacing
*Muscles buldge outward more

Those who became Level 2:

Goku: While training in Hell (DBZ: During the Anoyoichi Budoukai Saga/Beginning of Majin Buu Saga).
Gohan: During the fight with Cell (DBZ: Late Cell Saga).
Vegeta: Vegeta turned SSJ2 when Babidi possessed him, He made it perfectly clear that being possessed raised his power level immensely (DBZ: Between Cell and Majin Buu).
Gotenks: This is never really noticed... probably either did it after fusing or during the Super Buu fight (Majin Buu).

Super Saiyan Level 3
Here is where some big power comes into play, Level 3 has not been achived by many, and for those who did, thier power shoots up in a major way.
Differences between level 2 and 3 are...
*Hair grows down to thier legs
*eyebrows are lost
*Out grow thier clothes, muscles buldge bigger then before

Those who made it this far:

Goku: While training in Hell. (DBZ: During the Anoyoichi Budoukai Saga/Beginning of Majin Buu Saga).
Gotenks: Achieved during training in the Room of Spirit and Time. (DBZ: Majin Buu Saga).

Super Saiyan Level 4
Here it is, the top of the ladder for Super Saiyans. These guys pack serious power.. and they very easy to pick out from the rest of the other levels.. here is why..
*Eye color is green, red & brown
*Hair turns back, and grows only to its orginal state
*Chest grows red fur
*One heck on an attitude problem

Who achived the ultimate form?

Goku: After Goku becomes Oozaru in a Super Saiyan state (GT: During the Bebi Saga).
Vegeta: Courtesy of Bulma's Brute Ray, he was able to become a Golden Oozaru and then shrink down. (GT: During the Bebi Saga).

Super Saiyan Level 5
Ok.. I will put it as simple as I can.. there is no level 5. All the rumors are false, bottom line