About ten years ago, I bought my first stack of QSL cards. They were simple card stock with black ink printing. Nothing special. Seven years later, I had only mailed around 25 of the 500 that I got in the order. I moved and with the change of address, the cards were pretty useless to me. After several years of bouncing around apartments, I have a home now and an actual HF station set up. The recent 10 meter openings have sparked a renewed HF interest in me. I just ordered a new batch of cards from The QSL Man a couple of weeks ago. I read reviews and him and the majority were happy with the cards, so I am kind of excited to see them, when they come in the mail.
As far as a favorite. When I was younger, I had an S-10 pickup. I was working at Radio Shack and we started to carry the HTX-10 10m mobile. I picked one up on sale and with my 25% discount. That, along with a Larsen 10m NMO mount antenna, I worked the world. The day I got the antenna, I was trimming/tuning it down from the 25MHz length it shipped as. Once I got it tuned, I called CQ. The first station to come back was in the Azores. I sent a card. Three years later, a "buro" envelope showed up in my mail, with the return card. I really like that card for several reasons.
To me, there is nothing cooler than getting a hand written (or even printed) card that was actually filled out in the person's home and they took the time to mail it to you. Not only did the radio waves travel that distance, but so did that card. I don't like eQSL cards for this reason alone. It's just not as personal. It doesn't take as much effort. I'm all for the classic QSL method!