My favorite is Fungal Folly. It’s the first one I did when I got to play in beta for a while. It seemed incredibly, crazily over-tuned w/mobs that hit very hard and had a ton of health. We were given good gear in the beta that we could grab from a vendor, but I don’t know what the scaling was between the gear and the Delve, itself. It seemed pretty crazy, tbh. Wasn’t what I expected from a “level 1”. lol
At one point, I looked around the delve and I really liked the art/aesthetic of the place. Dark but w/vibrant colors. (A fishing hole??)
I still love the mushroom-guy noises. X) They’re so funny.
And the music. I stopped a few moments and listened to the music. It’s gorgeous. It’s my favorite music in the game for this expac (along w/the main Hallowfall theme that plays in the WoW Log-in.)
When TWW came out, I felt I had a personal grudge w/this delve! Dang it! So I ran it a lot and it wound up becoming my favorite.
I really like Delves. They’re even more fun w/other people, because the atmosphere of most groups is pretty chill; people seem to actually communicate and joke around a bit more. We’d point out if we see a treasure or curiosity to grab. It’s interesting to see how people will play in delves as they may take a different approach to what I’ve been doing; I learn a few things. 
I actually really like having Brann there. I’ve been a fan of his since he started coming with us into dungeons in Wotlk. I set him as a healer and just back-in or straife into his healing potions. (He usually throws 'em right near my feet.) He also has a big glowy-looking ability that heals well. I dps, but Brann still does some, too. – I like hearing his banter. xD – I know some folks don’t like having Brann around as they say he can pull stuff or other issues, but I haven’t had this experience. I’m just real careful where I go on my character. My main-alt is a Hunter, so I’m used to having a companion and I kinda like that.
S’fun.
edit: Ah, yah, the big Spore swirlies. Gosh, those can be annoying. lol xD I got really, really good at avoiding them. Some groups use the swirlies to damage the mobs, yet I still tend to just avoid them, completely.
I also really like the kobold one w/the lit candles, trying to keep the candle lit.