In order of decreasing strength:
1) Chopped CF
2) Milled Glass/Colloidal Silica
3) Microballoons
In order of decreasing ease of use
1) Everything except chopped fibers
2) Chopped CF (or chopped glass or kevlar)
In order of decreasing ease of sanding:
1) Microballoons
2) Milled glass/colloidal silica
3) Chopped CF
Basically, use chopped CF for maximum strength at the expense of the ability to sand it and the ease of use (it becomes a hairy, goopy mess when you add chopped CF to epoxy, but it's incredibly strong). Use milled glass and colloidal silica for a good balance of strength and ease of use (they're a good choice for things like external fillets). Microballoons severely decrease the strength of the epoxy, but they also make it a good sandable filler. So, use them if you need something as more of a filler, rather than a strong adhesive.
In addition, with some of the fillers (colloidal silica is my favorite), you can thicken the epoxy to the point at which it will hold its shape regardless of orientation. At this point, you can shape beautiful fillets and not worry about running or sagging. Not only that, but you can do all of your fillets with 60 min or slower epoxy, and you can still finish all of them at once in 15 minutes (and then wander off to do something else while they cure).
