DS9 is still my favourite of all the spinoffs. This is, as you point out, because it actually has a plot (several, in fact), and develops this plot pretty successfully and consistently over the series. All the other trek shows were very episodic, with nothing much ever happening that affected future events or referred to past ones.

Babylon 5 is like this as well, but even more so, to the point that you pretty much HAVE to start at the beginning and watch them all in order or it doesn't make any sense. Farscape also both suffers and benefits from this.

Enterprise is in some ways fairly plot driven in its own right, but definitely suffers from pretty bad writing much of the time. It could be so much better, which is the depressing thing about it.

pca