University Days is amazing. I'm so mad that it's been on such a long hiatus.
UD gets pretty saucy too. I'd suggest "The Fires of Friendship" and it's followups. They're higher in quality and equal in similar content to UD, so I feel like you'd like it a lot. Heavy Rarijack.
"Who is this Lord Tirek you Speak Of?" is hilarious and zany. Comedic Human in Equestria.
"What I've Done" and it's sequel if you enjoy Humans-in-Equestria type stories + SciFi.
"Poniocracy" for the same reason as above, except replace drama and suspense with hilarity. Absolute hilarity.
"Duet in the Folk-Style" is a really good romance fic.TaviMac.
"Becoming Whole" is great. Chrysalis drama post-Season 2 finale.
"This Game of Mine" is awesome. Sweetie Belle + Video Games. Very dramatic but homey.
"The Striped Pony". Feel-good Zecora story.
"The Irony of Applejack" is a super huge drama trilogy. Really good if you're into Ajay. Changeling-all-along fic.
"Carry On". Post apocalyptic Equestria + SciFi.
"The Whittler". Human in Equestria. It's still very young. Don't know where it'll go.
One of the biggest purely fan made, no crossovers whatsoever, fanfic out there is "Antipodes" and it's sequel "Lodestones." I've been meaning to read those two but it's difficult to start.
Those are all stuff on my favorites list that I love.
If you want any more suggestions, hit me up.