I was trying to think of a top five list of active superheroes who have never joined a real team (by "join a real team," I mean, like, being a member of the Secret Defenders or the Justice League Task Force for a mission wouldn't count), and man, past Daredevil, the pickings sure are slim!
How many purely solo superheroes in Marvel and DC Comics can you even name?
Black Crow? I guess, through a loophole, Spider Woman (Jessica Drew)? Spider Woman (Mattie Franklin)?
And in DC Comics, they don't even have a Registration Act that makes a lot of heroes automatically part of a team, and yet still it is difficult to identify heroes who are purely solo ones - Red Robin count, yet?
The All-New Atom counts, but both of them are so new that, well, it is a bit of a cheat (Blue Beetle, however, is just as new and he is already on a team).
Adam Strange, I guess, counts - he's pretty big. I guess he'd be right behind Daredevil in terms of significant heroes who never joined a team. Max Mercury? He's not active, though...
It is amazing how every hero seems to ultimately end up on a superhero team...