Probably between The Mummy at UO or Space Mountain at MK. Mummy is already a great ride but the restraints are just amazing. Because you can hold onto it as well as having it hold you in for the banked turns. As for Space, I like it better than BTM because you do not have to sit next to someone. Also you have almost nothing to hold onto. I don't really like the far back though, but you get the most amazing air time on B side. But they can really hurt..
Most uncomfortable: Tie between Big Thunder Mountain, Expedition Everest and Forbidden Journey(I wouldn't count it as a coaster but still..)
Starting with BTM, it is never tight and also it hurts to get thrown into if you sit in the back. (Best seat+ the BTM trick only works in this seat.) SM's lap bars can do the same but they are a little more padded at least. Also I think that BTM should get clamshell lapbars. Of course that means that both parents can't sit with their child. But it is a little safer for both parties and more comfortable.
With Everest, it's really just a height issue with me. I am very short so I have short arms as well, so I hold onto the lap bar. The actual thing you can hold onto is way to far away from any guest that is shorter than 5'3. You have to stretch so far if you are short lol. Anyway, I would probably fix this with lapbars that have something to hold onto. (Everest already has Vekoma clamshell lapbars.)
Last but not least, HPATFJ Or Harry Potter and the Forbidden Journey. I just really didn't like this one because it felt claustrophobic. I guess because of the horseshoe(That is what I call them lol) being so big along with the top being covered (so you do not see the projectors) it is just not comfortable. I do like the "Handle bar" thing, it is much more comfortable than normal "horseshoe" handlebar things.
That is just my two cents though ^^