I`ll look for quotes tomorrow, but as for ratings:
Brian - John rated him as his favourite instrumentist in a 70s magazine in Japan. Fred commented he had a very different style of songwriting and Mercury hardly participated in his songs (at least during 80s, and he meant on arrangements and stuff), and commented that he trusted in his guitar sound as much as in an orchestra; Roger said he was a typical guitarist in the matter that he didn`t know drums have to be tuned, because guitarists think of drums as just things you hit
Freddie - Brian once compared how his own songs were harmonically simple while Freddie`s were in weird keys and had tons of chords; Roger commented that Fred was an amazing musician and people tended to overlook that because of the ridiculous costumes and stuff: John rated him as his favourite singer in that Japanese magazine, and commented he was the one who spent most of time at the studios during Queen II, and in general with the songs he wrote
Roger - Brian said he was an amazing drummer, Fred said he could trust him as well as a machine, and that he (Freddie) participated a lot in Roger`s songs from an early stage, and sometimes even took them over, but they were still Roger`s songs
John - They all have said he`s an amazing bass player. Roger did say "he`s a great bassist, for a singer".
John hated Hot Space. Fred's favourite singer was not Paul Rodgers. Roger didn't compose 'Innuendo.' Witness testimonies are often inaccurate. Wales is not in England. 'Bo Rhap' hasn't got 180 voices.