All Endings
Under 2 hours with 100% - Samus with suit off, standing up with a clear image of her to the left. Best Appearance

Under 2 hours, under 100% - Samus with suit off, leaning up a ledge with head leaned back.

Over 2 hours, 100% - Samus with suit off, in a city.

Over 2 hours, under 3 hours, under 100% - Samus with helmet off.

Over 3 hours, under 100% - Nothing Special. Worst Appearance.

After Game Goodies
After you beat the game (no matter the time), go back to your game file WITHOUT TURING THE GAME OFF! when a menu pops up, select "Start Over". hence the opening movie, press start. you'll notice a timer in the bottom of the screen displaying elapsed time of the game and counters of Missile Tank Updgrades, Energy Tank Upgrades and Power Bomb Upgrades. also in that sector you are in, on the map in the start menu, it will show you if you have collected all the items in that sector.

Samus without her suit:
Successfully complete the game with a time under 6:30 and all items (100%) to see Samus Aran without the suit, wearing something similar to a bathing suit.

Secret Ending
Beat the game at 100% in under 2 hours to get the secret ending.

Gallery Mode (Japanese version only)
Just beat the game once in any difficulty mode to unlock Gallery Mode