definetely international foxtrott. I think it's very difficult to reach a reasonably good standard, but i guess that depends on your definition of it.
Also, maybe it's just because if you do it wrongly it feels bad, whereas other dances can feel ok even when you do them badly...

As for vienniese, it's difficult, but I think most people (including me) have most difficulties with it because they're scared of it. Fright alwyas means tension, and then you're screwed... :P

Of the dances I enjoy I think for me that Salsa is the trickiest with its patterns; I forget most of what I learned in classes - lol.

"Dancing is a contact sport; football is a hitting sport" - Vince Lombardi

Vienese Waltz still freaks me out. I'm a little afraid of it.

Strangely, no other dances indimidate me whatsoever.

I guess I imagine that the dancers get incredibly dizzy...

For me standard Tango - basic is easy, but I've always had difficulty with more intermediate patterns. For this reason I have not been learning any Tango.

"Dancing is a contact sport; football is a hitting sport" - Vince Lombardi