Just spotted this thread! I'm a huge Chicago fan - particularly of the amazing bass playing/vocals of Peter Cetera (hence my name on here). For me, he took the best elements of Motown and McCartney basslines and progressed them further in a jazz-rock environment, aided by the fantastic Danny Seraphine on drums. Add the much missed and hugely underrated Terry Kath on guitar (and Ray Charles-esque vocals) and you had imho the FINEST power trio ever in rock! Yes, better than Hendrix/Cream etc.....
Further add all the great songs and a kickass horn section and you have the now legendary Chicago 'sound'. Just fantastic!