I love 4x5 contacts -- something about the intimacy. They remind me of fine jewelry, small and intricate. A little hard to describe (and I'm a little incoherent after a silly day at work) - but they make me work a bit harder (in a good way) -- I find myself digging about trying to see "everything".
Close second is 5x7 -- I've been shooting with an Anba Ikeda, light as a feather. And I love the aspect ratio. I also have an old B&J that I got for a song and am restoring.
Some day I want to learn Pd -- I'm hoping my lab science background is helpful!
But beware the pull of "bigger" -- I stumbled on a stellar deal for a user Deardorff 8x10, just had to buy it. 😉
JT