Which army?

I agree with the above posters. Choose the army you most like the look of.

That aside, it also depends on what type of army you want to play. A sturdy gunline (Dwarves)? An elite army of specialists (High Elves)? Or a resilient close combat army (Warriors of Chaos)?

If all you care about is competitiveness, then Warriors of Chaos are probably the most competitive, just because of how cheap marauders are.

Both Dwarves and Warriors of Chaos have very strong and sturdy close combat units, supported by strong heroes/lords. The main differences between the armies are:

1. Dwarves have a lot of shooting, whereas Warriors of Chaos have virtually none.

2. Warriors of chaos have a good number of fast units and spellcasters (mainly offensive/attack magic), whereas Dwarves have virtually none of either.

Either way you're basically giving up something: either shooting or magic. You just have to decide which you want more.