I loved Annie, George, Mitchell and Nina, but I've also grown to love Hal and Tom very quickly, and while we saw little of Alex, she seemed like she could be very interesting too. The three of them have great chemistry.
Being Human has proven itself to be really great at creating engaging characters. I like that the new characters are not copies of the old ones at all, they are all very different but lovable in their own way.
The only thing I didn't like about season 4 was the baby storyline.