The following is a fully playable character, or ally, or enemy (if you really hate your players).
Basic Attributes ST 9999 ; DX 40 ; IQ 10 ; HT 10 
Secondary Characteristics HP 9999 ; Per 10 ; Will 10 ; FP 450 ;
Basic Speed 375 ; Basic Move 375 
Other DR 0; Dodge 378; SM 1; sw 1003d; thr 1001d
Advantages Ally (Genos) (Appears almost all the time) ; Ally (Mumen Rider)
(Appears quite rarely) ; Enhanced Time Sense ; Luck, Ridiculous (Defens-
ive) ; Regeneration (Extreme) ; Reputation 2 (Recognized occasionally) ; Weapon Master (Unarmed) 
Disadvantages Dependent (Cactus on windowsill) [-3]; Enemy (Speed-o-sound Sonic) (Appears fairly often; Rival) [-2]; Reputation 3 (Class B Hero, Recognized occasionally) [-5]
Quirks Ennui about boring life [-1]; Sensitive about baldness [-1]; Trademark (kills in a single punch!) [-1]
Skills Brawling 40 
Designers notesThe first challenge for designing Saitama was where to limit the power of build. GCS maxed out at ST 9,999 so this seemed a good place to stop. This left his default damage much lower than I'd like (at one point he punches an asteroid to save the city, which traditionally takes a nuclear explosion with ~ 6d x 90,000, but this build has default thr damage around 1000d)
HP was interesting. In the show, it's never clear whether Saitama is just very very strong or if he is truly invulnerable. Since he doesn't seem to slow down even when in a fight for a long time, I opted to give him Extreme Regeneration and let his HP 9,999 carry him pretty much all the way. (Side note: with such high HP, Saitama essentially never suffers shock or a major wound)
Basic Speed is set at 375 as the speed of sound is 375.25 and Saitama doesn't cause sonic booms everwhere he goes. This also gives a dodge of 378, but effective dodge of 16 felt too low, so Saitama has Luck to compensate.
DX is set at 40 because higher seems ... futile? At this level he should be able to deal with even -20 modifiers without breaking a sweat!
HT and IQ are set to 10 because apart from super speed and super strength (and endurance, more on that later) Saitama seems like an ordinary guy. So much so, that when he and Genos had an eating contest, he couldn't finish without vomiting!
FP is tuned up to 450 because in the last two episodes, he has basically an extended fight with lots of monsters, followed by a huge fight which lasts at least 25 minutes with a single opponent basically without slowing down. If we assume he loses 1 FP every 10 seconds, he needs around 300 FP to make it to the 1/3FP cutoff for slowing down.
Allies are pretty standard, the high point build of Saitama means that basically everyone is ~ 1 pt enemy or ally.
What did you think of this build? How would you have done it differently? Let me know in the comments below!