Literally ever online site, magazine and youtube channel puts vision as the no1/2 candidate for death. Iron Man, cap, nebula, Loki also appear highly on a lot of lists.
Well, those aren't exactly unsound choices, both plot-wise and contract-wise:
Vision - Plot-wise he's in immediate danger because Thanos needs what gave Vision life in the first to win.
Loki - Some would argue Ragnarok already completed his main arc of true redemption in the MCU and while you can tell more stories with him, who knows how much more time Hiddleston will want to keep playing a big role. Plot-wise, he's probably on Thanos' naughty list after the fiasco with the Tesseract and would give Thor a personal motive to fight the titan (I recall hearing the Russo's say he had an improtant role in this movie).
Iron Man & Captain America - They're arguably the two most important pillars of the (current) MCU so plot-wise killing off one of these two characters would be the biggest shock for the audience (much more than killing, say, Falcon, Rhodes or even Vision) and cement the stakes. RDJ and Chris Evans have also gone on record that they'll probably be leaving after A4 (I think, if they truly die, it'll be in that movie then).
Nebula & Gamora - Plot-wsie, they're the two characters with the biggest connection to Thanos. Their death at his hands would feel personal and, if paired with competent characterization, give depth to the villain. Nebula's arc seems compeltely centered around Thanos (which probably means her story would be difficult to continue without him) and Gamora's called her favorite daughter.
We also have to keep in mind that any "newbie" death, especially of a character with franchise potential, will feel fake. So people like Panther, Strange, Spider-Man or even Thor (who's just found his groove) are probably safe. And secondary character deaths probably won't have the same impact, even if they're fan favorites, because this is supposed to be the "culmination" of the MCU and the climax of a story shouldn't be the death of a secondary unless that secondary is at least as important to the story as the main character (something I don't think has really happened in the MCU).