Splash Mountain always seems to be my biggest line. I have been fortunate to have gone to WDW in a lowish season so long lines havn't been to much of a problem, have walked on many a time.
Our fastest at DL was walk on at Space Mountain. We got FP for ride and saw the Stand-by line said 20 minutes. We decided to get in line and it turned out to be a walk on. Rode Space then rode again with our FP, again a walk on. Our fastest at WDW was The Living Seas, a walk on.