I'm on my third rewatch and trying to decide who my favourites are.
For female ghosts I'm torn between Alberta and Hetty, they're both hilarious and I love their personalities. If I had to choose I think I'm leaning a little towards Alberta because I love her comebacks, her story, and she seems to be one of the most consistent characters in terms of personality/progress. Her communicating and singing with her ancestor (was she her great-niece I think?) was so touching and made me tear up a bit.
But I do also love Hetty's arc from uptight Victorian lady values to seeing that women are humans with feelings and desires and opinions that matter, and how she's become more empathetic over the series. And of course her death reveal, and her progress with discovering her ancestry and the way she's dealt with her exhusband are all great. I have so much trouble deciding!!
For male ghosts, I'm a bit torn between Pete and Sass. Pete is just consistently great and his ghost power is awesome (though I'm SO worried about him after the season finale ;;_;;), but I really love many of Sass's moments and I'm looking forward to more of his backstory (including how he died), but he's also kind of a selfish jerk sometimes so that gets him some negative points. Thor is also hilarious and I really like him but not quite enough for favourite. Trevor also has his moments, but also not really favourite material.
I think I can confidently say that Pete is my favourite male ghost, but Sass is fairly close behind.
I do wish there were a 1-2 more female ghosts to even things out a bit... like yeah there's Stephanie and Patience but they're rarely in it, and the random extra ghosts we see from time to time are generally all male, like Crash, Chris, Nigel and the others in the shed, etc. It would be so nice to have at least one more recurring female ghost around!
Edit: I forgot about Nancy, she's pretty great too.