Ok what is most uncomfortable, challenging 4 string (maaaaaybe 5) bass to play... The bass that bites back, that's not handed to you on a silver platter, something you have to work hard to play, like a bull trying to throw you off it's back. A bass that is just... A WILD BEAST WAITING TO BE TAMED! NOTE THIS: IT HAS SOUND GOOD! IT SOUNDS FANTASTIC BUT FEELS HORRIFIC! TRY TO INCLUDE THESE AS MUCH AS YOU CAN AND FIND BASSES THAT HAVE A LOT OF THESE FEATURES: heavy, lot of neck dive, fat chunky neck, little to NO body cuts, awkward string spacing, awkward hand placement (like pickup covers and little space to rest your thumb). Anything else? Thanks guys REALLY appreciate your digging!
The most unpleasent bass I ever owned was a little Hofner Shortie. It weighed hardly anything, but was simply unconfortable to play either standing or sitting. The body was too small and shaped wierd. It suffered neck dive and it's tone wasn't very good. I was very happy to part with it.
For me, the Hofner 500/1. Not exactly uncomfortable, I often play an acoustic bass, but it feels like I'm going to break it. Is it all in my head? Sure, but that doesn't change my mind.
The most uncomfortable bass to play I own is a RI 62 Jazz, I hate the flat neck, nothing to grab onto. I bought a new J neck and while it's a little thicker I'm still going to have to get used to it. I've played mostly P's although I had a P bass once with a 66 Jazz neck and I liked that one it was a little rounder and felt more substantial. I'm also thinking of either Berry Oaklying it or Mel Schactering it, not sure if I want that single coil sound of the Hagstrom Bisonic or the big fat humbucker from an EB-0, we'll see. It needs more big fat mids.
The most uncomfortable bass I've played had a string spacing of about 16mm, also anything over 9 1/2 lbs is uncomfortable to the point of being unplayable for me.
Ibanez BTB 400QM Has the widest neck I have ever encountered. So uncomfortable I flipped it after only 2 weeks.
A Dave Pushic custom I had built— the neck was thick but the middle of the back was totally flattened, like a bowl. I tried to love it but I had to sell it.