Skyliner and it's not even close. Every time I have been on the monorail it's been frustrating for one reason or another, two people sitting right in the center of the seat area so larger groups can't sit, or smells, or especially during the pandemic the chin diaper mask wearers that also have a hopefully benign cough. Strollers that people don't prep even though they get warnings we are coming into a station and hold up people from getting on or off because they just start getting things packed in when the doors open.
I don't see the monorail as iconic or special, but even if I did the skyliner would still be a better method of travel in my view. They even have special loading and unloading for wheelchairs and strollers to get around the main issue with the monorail.
The only thing about the skyliner I am not thrilled with is the possibility of it stopping and having to hang in the sun for an hour or more. But I can only give it a slightly worse score than the monorail on that front because it's not like you can always just hop off the monorail either if it stopped half way along the route. But the skyliner is at least a little more sketchy on that front.
In the end I am glad we have variety, and will likely use both. But in our trip in February we traveled by bus, skyliner, monorail, and Lyft. My ordered list of best to worst would be Skyliner > Lyft > Monorail > Bus. Bus would beat monorail if it wasn't full up, but almost every trip we had was nearly full occupancy. The monorail was surprisingly empty but we were also using it around mid-day when I think most people were already at the parks.