I think I'd have different answers depending on which difficulty.
I'm with most people, I think Argus was a really fun fight while it was still novel. However, I did expect more from Mythic after the awesomeness that was Mythic Archimonde in WoD. I was slightly let down in that regard.
My least favorite is Felhounds, especially on Mythic. I just hated the strat and the mechanics.
Coven's the worst...
*Looks at Norgannon*
*Cries*
The best? Argus.
He killed me. HE EVEN HAS A UNIVERSAL DESTRUCTION SPELL! D:
Best: argus
Worst: eonar by a landslide, and who the hell made the decision that run speed should decide who does the most dps!?
Best is Argus. He's just a solid end of expansion boss with some pretty cool mechanics thrown in to make the player more "powerful."
Worst is Coven by far. Worst boss in the expansion for me. Abilities tend to bug out, annoying rng, and way too long of a fight. It's like blizz had two bosses in mind, but decided to awkwardly mix them together.
Our entire guild hates coven.... we just started skipping it.
My favorite... probably Vari but I always have liked "patchwerk" style bosses.
I hated Aggramar and Varimathras on Mythic.
Argus was pretty fun just wish we could skip the RP.
I don't love or hate any of them. Argus is disappointing for an end boss, if it had the swirls from heroic it could've been more exciting like mannoroth-esque.
Aggramar is the only one that's a pain in the ass to pug, so that fight probably, because it requires the most coordination, which is what raiding should be about. Worst is probably Eonar. Glorified trash pull.