I choose the portraits I use based on aesthetics, but also very much so on emotes. I prefer portraits that possess emotes that are in good sportsmanship. I also don’t like portraits that are, frankly, annoying. Unsurprisingly, Sir Annoy-O is probably the biggest offender. Every emote this portrait has is designed to annoy, intentionally. Hello! Hello? Hello.
So, how do you choose your portraits? What are your favorite portraits? Why?
This is fun.
So, I choose it, I think, based on primarily aesthetics which appeal to me. How I think the hero looks best and what I feel fits the most. Then, I match a card back with each deck and hero based on the feel. I try to match colours, shades, brightness, etc.
Certain portraits even though I have them, I would never use. Like any of the pink or anime stuff they keep pushing on Jaina. Same with the beach theme portraits. Not for me.
Then any portrait that is Anduin I automatically detest because he literally has the worst emotes. So I just use Tyranndae (sp?).
Editing to add faves:
Medivh
Maiev
Alleria
Thunder King (how can dis be?)
1000 win warrior
1000 win Guldan
I like the idea of earning them so those are my favs, the ones you get for 500 and 1000 wins 
Sometimes I just go for what I think is cool or a new one I’ve recently acquired, but for the most part I try to pick heroes that I feel match the deck thematically.
The best looking woman for the class.
if it suits the style of the deck the best.
e.g. a fire heavy mage deck will have a fire-y mage portrait and so on.
Thematically, i.e. for a mid-range divine shield and “protector” -themed paladin deck, I’ll pick Arthas with Lordaeron coin.
Speaking of which, when will be able to choose the coin skin for individual decks, Team 5?
Only aesthetics. I block emotes every game. Most new portraits are ultra bad quality (Edwin was so bad it made me laugh) so I tend to use old ones.
I disagree. I believe they did a good job on the holiday bundle and the most recent Xuen bundle.
This doesn’t do it justice because it’s a png but you can probably view it in the shop.
I try to pick the skin that matches the more with the deck. An easy example is a Chaman Murloc deck, it NEEDS! to be played with the Morgl skin. And none of my decks can share the same skin.
The emotes.
It’s always the emotes.
Using Vashj for shaman and being able to say “curses!” Everytime I get a bad card or my opponent has a good turn.
i play paladin, now i look like one
Mostly I play the ones I like the looks of best. Some I’ll play if they were harder to earn.
I generally don’t pay too much attention to the emotes, but I did stop using Nemsy after a while because I felt like her “well played” sounded a little mean at the end of a match. That and pirate Garrosh got on my nerves pretty quickly. The dragon lady mage (why am I blanking on her name?) has a hilarious greeting, though.
What’s she say? I can’t recall her name either.
Something like “Humans sure do love their holidays–um, I mean of course we do.”
Spelunker Guff - because he is the nicest, sweetest character ever. I love him.
Red Riding Hood Jaina (for non-ping decks) paired with the baked pie card back - because that was one of my favorite Grimms fairy tales as a kid and Jaina rocks it.
Celeste (for ping deck) - obviously bc her hero power fires a lot faster.
Stalwart Cariel - because I really enjoyed her character over the course of the year and I think that’s the best one for her.
I like to match my card backs to the portrait as well and use the newest rewards track coin bc I play heros of both factions. I don’t emote so those don’t play a part in the decision, however some of the newer portraits have fun voice lines to go with certain cards played, mostly spells.
Her standard portrait says, “Wow! Well played!” In a tone that implies she’s being sarcastic.
Edit - I thought you were asking about Nemsy.
The dragon lady portrait is probably Lady Prestor but I don’t remember what she says, or it could be Celeste.
I want my emotes to be as BM as possible while looking as happy or smug as possible as well.
Well, that’s not very nice, is it? 
It’s Prestor. Apparently in WoW lore she’s a dragon (Alexstraza? Onyxia?) in disguise.