For MK, Pan is the top of my list for "must FP". Followers by BTMRR, then Space, then Splash.
BTMRR has a terrible queue in the summer. Same issues as Pan. Splash isn't nearly as hot, and really is a wonderful queue, as long as the line isn't back to the switchbacks...which it rarely does that. Space has a pretty decent wait until the very end...that last corridor before you board is brutal.
I don't mind waiting as long as it is comfortable...even if it's hot, if it's not overly crowded feeling (like Pan and BTMRR can get, I am ok with it.
Another terrible queue / wait, when it's hot, is Tomorrowland Speedway...that bridge with all the radiated heat from the pavement combined with how slow the line moves is just brutal!