Road Dogg easily. Billy was already a multiple time tag champ and was well liked by several higher ups. They pushed his athleticism much like they did Mr Perfects. Road Dogg was jerking curtains and doing dark match jobs before the NAO. The storyline with Rock-A-Billy and HTM was his first real gig since he was the Roadie and then "The Real Double J". At the time RD was not as well liked by a certain nameless group, while Billy was a hanger-on. Kind of like Aldo he was a guy the group didn't mind around but was not part of the core five.
When the NAO was formed, RD went from lifelong jobber/sideshow (Santino) to a tag team specialist and solid midcarder (kofi) while Billy was already there before that. No matter who he teamed with or didn't team with he was going to get the shot at the upper midcard like his KOTR crown, feud with the Rock, etc. He ended up not fitting there and dropped back down to another championship caliber tag team in billy and chuck.
He won titles and was consistently in feuds both before and after nao while RD had less success both before and after NAO than Billy. RD's career highlights are almost all while with NAO but Billy has numerous accolades before, during, and after NAO.
Billy
WWF Hardcore Championship (2 times)
WWF Intercontinental Championship (1 time)
WWE World Tag Team Championship (10 times) – with Bart Gunn (3), Road Dogg (5) and Chuck (2)
King of the Ring (1999)
RD
WWF Hardcore Championship (1 time)
WWF Intercontinental Championship (1 time)
WWF Tag Team Championship (5 times)
All of RD's titles came while in the NAO.... Billy had 6 titles and the KOTR crown, plus the highly promoted feud with the Rock without RD. Billy won as many tag titles without RD as he did with him, won singles accolades after, and so it becomes obvious that while Billy might have reached arguably the height of his career while with RD, he had a very successful career outside of RD. Forget the name and just focus on the stats without RD. 5 tag titles, 1 Hardcore, 1 IC, 1 KOTR, second biggest match at summerslam with the Rock beneath Austin, HHH, Foley with Ventura as ref. That's a solid career. Maybe not HOF worthy but respectable.
Through this examination it has been made undeniable that while Road Dogg needed Billy for any kind of success, Billy found plenty of success without Rodd Dogg. That being the case, the verdict must be that Road Dogg benefited much more from the tag team than Billy.