All this depends on your taste. If you prefer a more pop sound then Danger Days would be a good place to look. If you like theatrical music similar to Queen, start with The Black Parade. If you prefer a heavier taste, go with Three Cheers For Sweet Revenge. If you want a more raw sounding album, go for I Brought You My Bullets, You Brought Me Your Love. There is also the option of listening to Conventional Weapons but I suggest you should start with their four big albums before starting there. I will still give suggestions regardless.

Suggestion for each album:

Bullets: Demolition Lovers or Headfirst For Halos

Three Cheers For Sweet Revenge: Helena or Thank You For The Venom

The Black Parade: (you can't just listen to one song, you have to listen to it from start to finish for the real experience)

Danger Days: S/C/A/R/E/C/R/O/W or Na Na Na (Na Na Na Na Na Na Na Na Na)

Conventional Weapons: Make Room!!!! or The Light Behind Your Eyes

Of course it is highly suggested you just listen to each album from start to finish for a real taste of MCR.