Out of the birds that I have the most experience with, I would rank them (from easiest to care for - hardest to care for) Budgies, Lovebirds, Parrotlets.
My budgies have always been very low-maintenance and sweet, they never bit, and they were not too loud or messy. Very lovely birds!
My lovebirds are considerable louder than any of my budgies have been, and I have had 2 that were intent on emptying out all of the food from the food dishes, so that they could eat off the floor....so they were VERY messy! All of my tame ones are/have been cuddlier than the budgies, but would occasionally bite (not in a mean way, just when they were trying to step up or something and used too much pressure, but it hurts!), and my un-tame ones are not interested in people at all. However, my lovebirds all live in aviaries, so they are very entertaining to watch, and they have been more fun than my budgies.
Parrotlets are the hardest. My current parrotlet, Pistache, is absolutely wonderful, but very picky. She has an enormous attitude, if she is molting/offended/upset/in a bad mood she will bite (many times it is hard enough to draw blood), she gets very angry at certain colours/noises, she prefers women to men, she only likes people with long hair, etc. But on the other hand, when she is in a good mood she is the sweetest, most loving, best bird I have ever had! I love her to bits

And her actual care is quite simple: she's not too messy, not too loud, etc. It's just that parrotlets are so opinionated and fussy, so I had to rank them as hardest.
But really there is no 'hardest' or 'easiest' bird. All birds are different in their personalities, and every individual bird requires a different amount of care. And there are other factors, too. Like my lovebirds are (usually) pretty low maintenance, but their constant chirping would probably get to be annoying at times if I kept them inside.
@Mizzely asked some great questions - if you could answer those we might be able to help you a bit better. Best of luck to you finding a new birdie friend!