Fluttershy is, by far, the worst Mane 6. Boring, no character development(She grows a spine in one or two episodes but forgets about it immediately after the episode ends), and all she really does is whine and put feral animals' welfare before her friends. The whole "I'm shy and socially unadapted" gimmick has long since worn off, and the only perks Hasbro has given her (Discord, Flutterbat, Flutterhulk, Flutterguy) have been purely out of a matter of "What the fuck CAN we do with her?". For instance, Discord could very well have been reformed by Twilight or whatever. And don't get me started on Bats.
As for secondary characters, Diamond Tiara and Silver Spoon, as well as the other "background characters" you listed aren't background characters at all, so I don't know what you were smoking when you made that poll, breh. They have semi-established personalities and have numerous lines in numerous episodes. Like, wtf, brotato chip.
Anyway, I think we all can agree that Blueblood is an a-class doucher. Trenderhoof is pretty not great, too. Oh, and Maud Pie. Boring, uninspired plot-fodder. I really hope she's only a one-epsiode deal.
For actual background ponies, well, all of them. The fandom has ruined them with their little fabricated personalities, like that whole Lyra's obsession with hands malarkey. They're called "background ponies" for a reason. They belong in the background, just so Equestria isn't seen as a pseudo-wasteland with like 10 citizens. It's like... fuck.
Fin.