For me, the most miserable glider I've ever flown is the Schweizer 1-36. Flew a nearly brand new one, thought it horrible. The pitch trim system was the main problem. It was much too powerful and made the stick very heavy. I chalked it up to being new and tight. Flew a different one thinking it had to be better. Nope just as horrible as the first. A few year later, found the mangled remains of that first 1-36 in a repair shop a long way from where I flew it.

Someone mentioned the TG-2. Flew one of those on a 60 mile ferry tow. If you think the 72 mph redline made cross country soaring tough, what about flying the thing for 60 miles on tow behind a towplane that could not tow that slow? I was never so happy to get to a reasonable bail-out altitude. The split ailerons on this one had been covered as single units and would bind a bit, resulting in limited movement and poor roll control. No rudder pedal adjustments and a fixed seat made for a very cramped seating position. However, after reaching the destination at 5000 agl, I found that the thing would spin and recover very well due to that huge rudder.

I think Kirk mentioned the Schweizer 1-23. The D models and later are probably the best gliders Schweizer ever built in serial production. One of my favorite flights of all time was in a 1-23D. Climbed the upwind side of a tall cu from it's 5000' base all the way to 8000' agl, then flew the cloud street like a ridge for an hour. Also had my first outlanding in a 1-23H15.