By "monster creation", do you mean "creature creation"? I tend to view characters, NPCs, and monsters as being all pretty much the same. They all share the common aspect of being creatures that have a race, with possibly race levels, class levels and templates added to them.
The
best one for an individual, is the app that's meets
their needs the best.
What are you looking for, from the following?
1. Adding class levels to any basic race.
2. Add equipment (random generation?).
3. Add skills, feats, and magic spells (random generation?)
4. Add more race levels to the standard creature.
5. Add templates.
6. Ability to create more monster types.
7. Calculated statblock from all race, class, equipment, feat etc. modifiers.
8. Add in-game calculations on top on statblock (eg choices for Expertise points, Power Attack points, option for Monk BAB, option for Rapid Shot, option for barbarian rage), and conditions (eg. Blinded, grappled).
Any other points?
I don't know much about CS, but the apps I'd consider relevant here include E-Tools, PCGen and RPM (which I develop).
[Step in and correct me where I get it wrong guys

]
E-Tools would probably have strength with accuracy of a massive race database, and fall down badly if you needed templates. It also has the benefit on being the only app "getting away" with providing non-core material online (Wizard's copyright-protected).
PCGen wouldn't be particularly friendly to adding new creatures (of course there are a lot who have learned the "lst" lingo, and now find it easy).
RPM alone does the options stuff in point 8 (why I added it, of course - but hey, it's impotant!

). Its of most use if you have a computer at the gaming table.