Fact of your avatar and Serana is a vampire makes this post rather amusing...

Anyways. I am unable to vote due to no Derkeethus. He's my favorite follower and most useful one I've had.

Kharjo became my favorite when I was involved in a skirmish and started hearing fireballs getting spammed. I assumed this was from an enemy mage, but suddenly the enemy in front of me disappeared, and I realized the fireballs were coming from Kharjo, wielding a staff of fireballs that I didn't even give him. (At least I don't remember doing so--perhaps it was in a chest I asked him to loot.)

Later I gave him a staff of ice storm and next thing I knew he was dual-wielding the staffs.

Where's Mjoll? She's one of my favourite followers, though I do have a soft spot for Kharjo.

Cicero is just a weirdo, can't stand him.

I'm pretty sure it was one of the official patches. Maybe 1.7 or something like that? It might have been one of the patches that was released before one of the DLCs - I know that voice actor recorded new lines for Dawnguard (she does Sorine's voice) and I think she did new lines for Dragonborn too, so it's possible these lines weren't in the game from the start but were recorded later. But I'm not sure. In the last game I used her, she had three different answers when I asked her to trade things: "I'm swooorn to... " (Yes, we have all heard it by now!), "How can I serve you, my Thane" (or something very similar, and she doesn't sound sarcastic), and finally one more neutral, I think it is "what do you need?". And that last one is most likely the same that also is used by Annekke and any other followers with the same voice actor.

I think it depends on what you're doing, what you're fighting, etc.

Against dragons, shock and ice mages rule all. My ideal Blades team would be Marcurio, Onmund, and Illia.

Against the various enemies of Solstheim, I would want warriors. Vorstag's served my Redguard well, and Lydia seemed OK the short time I used her.

A close-in warrior might want a mage or archer for backup. An archer or conjurer who avoids close-in stuff might want a warrior to protect them. Or, a mage might like fighting alongside another mage.

I think that your playstyle and most likely adversaries will determine which follower you like.