I'd go Corgan, Thayil, Cantrell, and then McCready.
Love him or hate him, Corgan is just a technical marvel. I'd say the same to a slightly lesser extent about Thayil, due to his creative usage of alternate tunings plus his proficiency. Cantrell is an excellent guitar player/songwriter, and I put him behind Thayil only really based on preference of styles.
McCready goes last for me for a couple of reasons, but the biggest is that back in the day, he never wrote any of the songs, just wrote their solos. This may have changed somewhat in more recent years, but back when Ten and Vs. came out, I just feel the other folks on that list were accomplishing more. Not really knocking his abilities though, he's a great live performer and puts together some great solos (sometimes on the fly!), so it's easy to see why some would take him higher on the list.
