this is a matter of trial and error. Almost any effect box can give you from horrible to cool. You need to play with them and see what you like, and to which tune the effect would fit.
I have tried the octave box (which I nicked from the guitarist in the late '80 (the time of the crime, not the age of the guitarist):shocked, and that made some cool effects on a couple of ballads we played.
I know Dooce has had some experience of the Digitech Vocal 300. He lent it to me for a while and I have to say that, in the short time I had it, I got very attached to it. All the things you want and more!