Mr Incredible was an obvious target for Buddy's fan-affection for two key reasons.
Visibility / Merchandising
He was the most visible and celebrated of the Supers. He was both Time and Life's "Super of the Year" and repeatedly saved the city, earning the 'Key to the City', numerous awards for gallantry as well as the 'Civilian Medal of Honor'.
Unlike other Supers he seems more than happy to endorse pretty much anything, including crappy dolls, crappy spoken-word albums and even crappier animated features, not to mention branded cereals and costumes.


Fan-club
Mr Incredible seems to have taken particular pride in interacting with his fanbase. In addition to attending fan-events, he's done photo-ops, interviews, autograph signings and seems more than happy to bask in the publicity that comes with his achievements. He even has his own "Mr. Incredible Cadets", a group that presumably emulates the scouting movement.
For someone starting to take an interest in Supers in general, he would be the obvious choice if you wanted to get 'up close and personal' with a real live one.

BOB (MR. INCREDIBLE): Look, I've been nice, I've stood for photos, signed every scrap of paper you pushed at me but this is...
BUDDY (INCREDIBOY): No, you don't have to worry about training me. I know all your moves, your crime fighting style, favorite catch
phrases, everything! I'm your number one fan!