Yeah Shadow does indeed dominate Tails in the popularity department but Knuckles doesn't.
Oh and for your information Knuckles is considered Sonic sidekick,Tails is pretty much Sonic oldest friend and nothing more.
To end this i quote Knuckles' Sonic Next-gen official profile:
"Sonic's famous sidekick spends most of his time alone on Angel Island guarding the Master Emerald, so he's always happy to help Sonic out – especially if it involves scrambling Dr. Eggman's evil plans for world domination."
See Sega has degraded Knuckles,i am not making it up,Tails is more popular then Knuckles face it you can't fight facts.
The Poll pretty much answer it:
Sonic > Shadow > Tails > Knuckles
http://sonic.sega.jp/event/016/index.html