That question reminds me of the legend about a group of people who placed their troubles on a table and were offered the opportunity to choose between them. They all chose their own.
I would guess severe, chronic pain is the one thing hardest to live with, with depression a close second. Mental illnesses and dementia are probably the most difficult for family members to deal with. But, while most people with special needs would be happy not to have any, for many the conditions are simply part of their normal lives.
And there are positive aspects to having disabilities. For example, they help people appreciate the things they are able to do. One of the best things is the way people with similar special needs often understand and connect with each other quickly and easily because even if they have barely met they know a lot about each other's lives.