Metroid wouldn’t be Metroid without Samus, and Samus wouldn’t be Samus without her instantly-recognizable and trademark Varia Suit. It’s been featured in every single game, and much of the official promotional art. It was even in Metroid Fusion, where the original was worn not by Samus but by the SA-X (though Samus did wear it for the first few scenes).
So why is the Varia Suit so popular? Arguably, it’s one of the strongest suits in the game. A common—and important—feature shared throughout the series is that it greatly reduces damage, varying from 10% all the way up to 50%. Another important feature is that it negates temperature damage, mostly heat damage.
Not that that stopped Adam from forcing her to run through lava in Metroid: Other M without the suit. I’ll never forget that, Adam. Never.
Of course, much like Samus herself, the Varia Suit has had a few different iterations throughout the years. In the original NES game, the Varia suit is just a pink recolor of the standard Power Suit, the one Samus typically begins each title in. When Metroid II was released for the Game Boy, they added the token shoulder pads to differentiate between the two suits, as the Game Boy had a monochrome color palette. Understandably it’d be difficult to see a color swap if everything looks the same.
By the time Metroid: Zero Mission released, the Varia Suit had turned into the orange costume we all know and love. Nintendo also managed to decide on the name Varia Suit, as it was actually called the Barrier Suit in Metroid and Metroid II. Personally Varia sounds much better to me.
(Though if you want to just play as Samus in a leotard, be my guest and exploit the Justin Bailey cheat. But that’s none of my business.)