The Magitek Death Claw is my favorite of the mounts suggested so far. It has no clipping problems, is reasonably compact and not too flashy and makes sense both flying and on the ground. It's a bit physically inaccurate in steep ascents and descents though, as the character looks way too stiff, but that doesn't bother too much when viewing from behind.

Large mounts like Midgardsormr, Behemoth and Adamantoise usually offer a decent sitting position thanks to being too large to be sat astride, but their hugeness attribute restricts the places they can reasonably be used. They are best in open areas but don't work so well with smaller corridors or archways where they bang my character's head against the ceiling.

@Shadygrove do you mean the Eggshilaration System available on the Mog Station? From the picture it looks like it would stretch the skirt quite a bit and probably have clipping issues on the backside. For the other one it's spelled Korpokkur.