Well...they all have talent, but none of them is great all over. They really complimented one another, especially in terms of song writing and arranging.
Brian - Great song writer and guitarist, but not the best lead singer. Also needs others to turn him away from overindulging his insane guitar overdubs, harmonies, etc.
Roger - Great singer and good song writer, but his compositions often need some tinkering with to reach their full potential.
Freddie - Well, he is great all over, a great singer, song writer, pianist. But he needed the influence of others to keep from becoming TOO pop-ish at times, at least during the time he was doing solo albums. Tended to lose sight of the importance of arrangements of the instruments. Especially later in his career, with the synths and all, lost the idea that instruments were characters of their own in a song, and not just there to support the vocals. Brian was a good moderating influence here, just as Freddie was for him with his guitar stuff. Unfortunately, his great piano technique went to waste later in his career.
That's how I see it, anyway. All the members of Queen needed outside influence to reach their full potential. To curb and tweak their output.
On the other take, take someone like Elton John, and collaboration diminishes him more than helping him. Except when it comes to lyrics, of course...