I have never yet found any of the CK lines (Similar issues also with 2(x)ist), that have fitted me right yet.
However, I am stuck with C-in2, and they have never let me down yet, being it Brief, Boxer, Jock, Army Trunk, Punt, or Boxer-Brief, with the Bamboo / Zen range being the best.
Only line in C-in2 I would say to avoid, is the sling support versions, as for me I find the sling strap uncomfortable, though you can just ignore the use of the strap if needs be.