Truly, Sonny was about as subtle as a herd of cattle stampeding through a jingle bell factory. However, the belief is that Sonny would've been the most likely to be able to pull off such a feat out of all the Corleones. Tom might be a close second if his lack of Sicilian heritage didn't come back to bite him in the butt. Any other Corleone family member would've been marked out immediately as a spy and dealt with accordingly.

Sonny's saving grace for such a mission would be the precedent he created by his gaffe in the meeting where he made it abundantly clear to Sollozzo that he wanted the deal in spite of Vito's wishes. His cover would be that precedent, and Sollozzo would be loathe to whack him for potential espionage being that Sonny was his potential path to the Corleone resources that he needed.

If Sonny could hold frame for such a mission (and that's a big if at best), he could potentially bring some useful information back. He's certainly a better choice than Luca Brasi for that mission; Vito's cat would've been a better choice than Luca Brasi.