for two ropes of different diam., the "classic" knot is the double sheet bend, backed up by double fisherman's on each side. This is actually the knot used by some mtn rescue units, as it can be untied very quickly even after subjected to a huge (litter, 2 attendants, patient) load. Here's a picture:
This is a bit much for rec. climbing, but there it is. It is often taped up after tying to run through kootenays or over edge pro.