This question always comes around. And there´s just no answer to it.
If we could put birds ¨in a box¨ and say that X species behaves in Y way, then we could theoretically compile some sort of list. But even at that, we then have to take into consideration the human and how they fare. And I don´t mean experience, some birdy newbies do great, fantastic even with large macaws. Others have experience with pi´s or IRNs and then run into all sorts of problems while owning a lovebird.
It´s hard to rank species when each bird is so different, and our own skill and confidence is so different.
If all lovebirds were like my Sydney, I would recommend them to everyone. Such a perfect bird.... But I wonder... How would Sydney have been had he lived with someone else? Would he still be this perfect bird? Or would others not be able to train him right and rid the undesired behaviours (biting, mating etc).
If all lovebirds were like Aldora, I´d say there´s few people who would ¨enjoy¨ having them as companions. Not friendly, likes to do her own thing, very bitey, don´t mess with her. She´s a lot of hard work to live with, just cleaning her cage and sorting food is difficult. Oh, but she´s a sweetheart with my partner, like butter wouldn´t melt. She just isn´t that interested in being friendly with me, and I´m fine with that. But I know a lot of people looking for ¨pets¨ wouldn´t be happy with such relationship, or would treat her as less, which I would never do.