1) Captain America
If Tony Stark isn’t about to bite the bullet, then how about Steve Rogers? The de facto leader of the Avengers certainly seems more like the type to go down fighting. Captain Rogers is a soldier through and through, and the films have emphasized how he doesn’t know how to function in peaceful times. Dying while protecting the universe from one of those bullies he hates so much would definitely be the most fitting way for the character to go.
It also doesn’t hurt that there’s precedent for Steve Rogers dying and someone else taking up the Captain America mantle in the comics. In fact, both legacy Caps – Sam Wilson and Bucky Barnes – are rolling around the MCU. Bucky has got to be the favourite to replace his old best friend, though. Not least because Sebastian Stan will have a sizeable five movies left on his contract after Infinity War – which would presumably see him through at least Marvel’s Phase Four.
If any of the major Avengers were to die in Infinity War, it would make the most sense for it to be Steve. In the comics, Cap’s vibranium shield is even shattered by Thanos. You have to say that would make a heck of a visual and a great, symbolic way of calling time on his tenure as Captain America.