Barnabas was, IMHO, DS' most complex character.
Look at how many changes he underwent as the series progressed:
He started out an evil vampire with only two things on his mind--blood (cattle or human) and turning a sweet young thang into Josette. His single-mindedness in both endeavors was more than a tad nuts, as was Barnabas himself. Not so surprising, when you realize he was stuck in a coffin for so many years, unable to get out or find blood.
Barnabas becomes the Collins family protector, saving them from ghosts with evil intent (Quentin, Gerard), moving forward, backward and sideways in history to ensure their survival. Given that there was one point that he wanted to kill David Collins, the family's only Collins-named heir, that's coming a long way, baby!
Other complex characters in present day were Liz, David, Burke, Julia, Willie, and Roger. Sam, too, but to a lesser extent.
SPOILERS...
In 1897, we had Quentin, Judith, Count Petofi, Charity Trask/Pansy Faye, Magda, Jenny. All were multi-faceted individuals with very distinct qualities.
Jeb was complex, but ridiculous. His change from villain to hero was too fast and sloppily done. I never bought his love for Carolyn, although they did look good together--and she did marry him.
In 1840, Samantha, Gerard, and Gabriel were layered characters that came across as three-dimensional.
And yes, thinking along the same lines as my discussion about Barnabas, Angelique evolved from a vengeful witch in love to a far more complex woman. To give her credit, she tried to get over Barnabas and start a new life when she married Sky; too bad she fell for someone who turned out to be a Leviathan. Her childish efforts to make Maggie fall in love with Quentin really pissed me off at the writers, because I felt Angelique should have been beyond such petty devices by that time.
I know a lot of people don't like the fact that Barnabas professed his love for Angelique, but at the time it was first shown, it seemed appropriate to the fans. These two crazy kids were from the same era; both had suffered being supernatural creatures; both had grown as individuals.
Take from this what you will, and ProfStokes and Cassandra, excellent discussions on this topic!
Love, Robin