My favorite deck would have to be an old deck that my dad found at his work. It has a picture of an oscilloscope (the electronics his company produces) on the back and it is overall a really cheap deck. with zero quality. My dad gave it to me when I first started collecting and its more sentimental to me. Most of my favorite decks are ones that I receive from people I know and they aren't often great decks from popular companies. In turn that is why I enjoy giving decks to people I know from Kickstarter and other small companies. I find that these decks are more interesting to them and the appreciate the deck more.
Aside from those decks though, I really like the Jaqk Cellars, Sentinels, and S&Ms quite a bit. They are all amazing decks!! I'm also really looking forward to the many new decks coming out! With the upcoming Americana, Vortex, Revision 1, Seasons, etc... I could have a new favorite(s) soon haha.
... and a note about some of the decks that I love is the work put into producing it. I really respect and admire artists that put a lot of time and hard work into their art. The decks that are simply a different color or only have a custom back, ace, and jokers can be nice. However, it can clearly be seen from Alex's Vortex deck; Russell's Americana deck; Lance's Actuators deck; and Adam's Revision 1 deck all the hard work and time they put into their decks. All their hard work payed off and I respect that. (Sorry about the long post)