Thanks to the recent movies and thus boosted sales of comic books, Marvel's tin man is getting more fame than ever before. Despite being a 1960s hero, he was a good concept and continued on into the 2000s, In the movies, he largely maintained the hotshot executive-cum-mechanical genius image, though the setting of his origin was changed from the Vietnam War to the Middle East.
A science fiction novel that won the Hugo Award in 1966, Starship Troopers may perhaps have the first ever modern portrayal of powered armor. Because the "bugs" they fight are bigger and badder, they need the armor. The 1997 movie failed to portray the armor probably because of budget and effects constraints, but a 1980s anime portrayed the armor correctly.
HALO's Master Chief
From the game HALO, the star armored trooper's the Master Chief, so is the most well-known wearer of the Mjolnir armor. He is obviously the face of the series... even if his real face was never revealed (I think). This costume also drew inspiration (as many did) from Starship Troopers.
When it comes to the first ever scientifically powered armor in history, the most likely candidate may be the Lensman series. While the powered armor is at the same time a spacesuit, it is perhaps one of the earliest instances where a suit worn by the hero provides added capabilities. The Lensman series is composed of novels by E.E. Smith written in the 1930s.