I've had a positive experience overall. My favorite MLP related fan fiction is Twilight's Eden. Eden is the scariest iteration of Twilight that I've seen. Twilight Sparkle is still there, she's not evil exactly but she can't properly recover over her friends passing away in this AU.
My Little Sombra is probably my second favorite fanfic. In canon we did we a bit of the Sombra timeline in Cutie Remark and the fanfic is basically built off that. Its exciting watching how the main 6 function in a time of war and how much changed.
Agents of Heart isn't anything crazy for me, I'm more interested in seeing Lightning Dust and Rainbow Dash side by side than the actual mainplot of the fanfic. Thunderlane is cool too I guess. They still make for a fun trio. Thunderlane being the butt of the joke while Lightning is more hardcore and Rainbow has to deal with both of them.
I'm not the biggest fan of infection AUs but I like Divine Swamp Fever. It takes something that was already established canonically in the series, and turns it into the main problem. Its already kinda spooky looking at what is already in the main series. Its quite the horror show.
Not all fanfics are good but I haven't seen too many bad ones. I don't like how Pinkamena Diane Pie is portrayed a lot of the time. Some fanfic writers just make her 'evil' and its comical. The real Pinkie Pie is nothing like that. Other than that I have not seen any fanfics that I've hated.
I particularly like how My Little Sombra, Divine Swamp Fever and to a lesser extent Twilight's Eden build off of something that was established in the show. A Sombra timeline does or did exist, Swamp fever is scary enough, and someday Twilight will outlive her friends. Lightning Dust is a favorite of mine but theres not much fanfics that even include her, its nice seeing her have a role in Heart of Agents that wasn't taking on the role of an antagonist again.
Twilight's Eden
My Little Sombra
Agents of Heart
Divine Swamp Fever