To me, there are two sides of Jack's playing with Cream: in the studio, and live.
The studio cuts feature beautiful parts which he develops and varies brilliantly, yet sound so spontaneous... you can tell he's an improviser.
The live cuts feature no-holds-barred improvising which nonetheless has logic and shape... you can tell he's a composer.
"Deserted Cities of the Heart", "Pressed Rat & Warthog", "Those Were the Days", "Dance the Night Away" and "World of Pain" really demonstrate the aspect of his studio playing that I mentioned. Also, "P.R. & Warthog" has an instrumental middle section wherein he plays a long, Bach-like line that has always killed me.
"Crossroads", "I'm So Glad", "Sweet Wine" and "N.S.U." are my favorite live Cream cuts (for Jack Bruce).
You have to hear his solo albums, too, particularly "Harmony Row" and "Songs for a Tailor"-- the way he weaves his bass through the music is really something to hear. "Tickets for Waterfalls", "Victoria Sage", "Morning Story", "You Burned the Tables", "Boston Ball Game 1967" and "Escape to the Royal Wood on Ice" just scratch the surface.
Carla Bley's "Escalator Over the Hill" features a lot of great musicians, among them a trio made up of Jack, John McLaughlin and Paul Motian. Great stuff. (Jack also sings a duet with Linda Ronstadt, of all people.)
He's the first person I ever heard who made a bass "talk"... his playing has so much personality and intelligence. And balls.