For me it's got to be Roland D-110 - which is a shame because there are actually some interesting sounds to be had out of this LA cheapie with a little work.... The UI must have been designed by a mad man though!

Others rating poorly for seriously frustrating UI's (but not the way they sound):

Sequential Prophet 2000/2 samplers
Waldorf MicroWave I
Yamaha TX-802
I would also say all DX synths (bar DX1 & 5) but after some thought that's not fair because FM is never going to be easy to convey on a small screen and I think Yamaha did the best they could considering this. The UI on the SY/TG77 is actually pretty good.
And as a contrast - the best UI IMHO for such a tiny display has to be E-mu EMAX series - I can practically use my EMAX II's with my eyes shut - brilliant samplers!
