Depends on the requirement.
For VFR around Europe (where you obtain all the printed VFR charts and use the PC just for the plog generation, more or less) Navbox Pro is the best.
For UK-style IFR (messing about in Class G mainly, at sub-Class-A levels) Navbox Pro again because the planning is just like VFR; only the weather planning differs.
For airways, it is really helpful to have a program into which you can enter
A airway-name B airway-name C
and it draws the route, and generates a plog with all the individual waypoints (which is used enroute because ATC often give you shortcuts along it, and is used to load the route into a normal IFR GPS that doesn't understand airway names) the only software around is Jeppesen Flitestar or Jeppview.
The Jepp stuff does have an awkward user interface but if you want to see the ultimate example of poor design look at Jepp Flitedeck. However they have the most extensive data coverage and that's what you pay for.
If Navbox understood airways and plotted out the routes, a lot fewer people would use Flitestar.
The other nice feature of Flitestar is the jepp "raster charts" product which for about £200 gives you the 1:500k VFR charts for Europe. This again makes Flitestar relevant for VFR. Flitestar is the only "official" means of viewing these charts, and £200 is a lot less than the cost of all the paper ones.
Jeppview gives you a cut-down version of Flitestar for flight planning but most relevant is the approach plates.