Gigasampler.

Garritan Personal Orchestra (if you are a budget)

Vienna Symphonic Library (if you are not!)

Garritan Big Band

BFD

EZ Drummer (plus expansion packs)

Ivory

Real Strat

too many to mention... have you even DONE a search yet?

Sky's the limit.
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An arranger is just a tool. What matters is what you build with it..!