I don't have a ton of experience with budgies, cockatiels, linnies, or parrotlets, although I've met a number of great budgies and cockatiels and I like both species. But, I can tell you a bit about lovebirds and greencheeks, as these are species I know quite well.
Lovebirds, if obtained as handfed babies and loved a lot, are extremely sweet little "pocket parrots" who really do everything the big guys do, on their own scale. They are curious and active. I would recommend leaving them fully flighted, because it builds their confidence to be able to go where they want and it's easy enough to train them to go back to their cage. They don't really talk or whistle, but I think their lovebird chatter is really funny and sweet, if sometimes annoying. In lovebirds more than any other species, I have observed vast personality differences between the sexes. My male lovebird is a reserved, laid-back, cautious, nervous little dude and my female is a BOSS. She guards her food, she guards her cage, she lands on my head and bites my ear, and she thinks she is 10 feet tall! She will happily go toe-to-toe with one of my larger birds for a morsel of treat on the counter if she feels it should be hers. My grey is actually somewhat resigned to her and just sort of rolls his eyes and lets her be. My caique and my greencheeks though-- that's another story. I have to supervise carefully in order to make sure there is not a fight where she could get hurt because she sincerely believes she will win. I love lovebirds. But they need a lot of attention to remain friendly and will get really lonely if you don't spend enough time with them.
Greencheeked conures are awesome little birds and I think pretty good for newer bird owners. They are very sweet and forgiving of mistakes. My one little guy was accidentally stepped on by an ex-boyfriend when he was young and despite a broken foot and what must have been a terrifying experience, he remains a loving and interactive bird. My greencheeks talk a little-- "Chili" and "No" are about the extent of it. They also mimic a human laugh and will do so every time someone laughs. I call them the Peanut Gallery. One thing I will say about greencheeks though is that they are, in my experience, a bit nippy, so if you are really afraid of getting bit they are not probably the best. I mostly am able to ignore the nips because they don't hurt that bad or break skin, but they are little pinches that I notice and are a great deal harder than just beaking and feeling things out. You must be brave to own a greencheeked conure because they are brave and mischievous little buggers, which is fun, sweet, and a little scary.
