Fallout Equestria is amazing, just because its long doesnt make it bad, Its probably the second best Fanfic i have read, and the first is Fallout Equestria Project Horizons.
Its great and has a good plot, its long but only because it goes into detail about everything so you can get a feel for the universe that its based in. it does take a while to read each chapter though, but its completely worth it.
Oh and i cant really recommend anything because it depends on your taste of writing, i mainly read Ship fics because that's what i enjoy more, but yeah i wouldn't start off with any gore fics or anything long like Fallout Equestria. the first thing i read was my little Dashie, and its what got me into fan fics.