In a little more than a month, we will be witnessing the greatest crossover in the history of cinema when the entire MCU comes together to fight the threat that is Thanos. Which means a lot of serious shit is going down. Lots of people are gonna die, from both sides.

Source: preen

Now, while MCU characters have a habit of not staying dead - case in point, Agent Coulson -here's the thing. According to Screenrant, Kevin Feige, in an interview, said:

I mean, I could always list off the characters that we’ve killed in our movies that haven’t come back, but the big ones, which I know they’re looking at …? I would just say, yes. People need to be careful what they wish for.
Source: Babbletop

Yup. So who is it? Our best guesses are Loki, Iron Man or Captain America. While Loki has completed his storyline and deserves a grand send-off, the entire MCU started with Tony Stark and his death would bring the event to a full circle.

Source: Shipping

That being said, all available evidence point at Chris Evans' Steve Rogers. In the comics, following the death of Captain America, his allies, Bucky Barnes and Sam Wilson have taken on the mantle. 

Source: Wallup

Besides, Chris Evans, in an interview with the New York Times, had said, that his contract is due to expire after the 2nd Infinity War film and he doesn't intend on extending it.

You want to get off the train before they push you off!
Source: YouTube

So yeah, we hope that he doesn't die but he most probably will. And we are not ready for a world where Chris Evans is not Captain America.