My main DAW so far has been Apple's Garageband which seems to have a nice balance of simplicity and extendability thus far. I've just ordered Logic Express 9 which should be quite a bit more powerful.
Most of the shortcomings of Garageband are easily addressed with add-ons or plugins such as the incredible Crystal synthesizer. Using this to put together oscillators, envelopes, waves, filters etc. there are few sounds you cannot get together.
Added in to that are tons of software instruments and I've accumulated a library of 20,000+ sound samples so its pretty flexible for anything I might want to do. I've put together tons of songs over the years mostly located at Macjams.