For a beginner - you have some good choices there.
Sony - the "ES" Series of receivers are quite good, but Sony makes hundreds of lower end receivers and few of them stand out as being very good.
Denon has a great reputation and I could argue that it is 'the best' because of build quality, performance in music with demanding audiophile quality speakers.
But Onkyo TX-SR series of receivers have been quietly impressing a lot of people and made some "Best of" and "Editor Choice" lists last year. Nobody is praising the sound - but the number of features, connections and ease of use for budget prices has impressed a lot of magazines and people.
Onkyo is trying to become the new Denon or Yamaha so you get a lot of value for the money with the TX-SR receivers.
Let me suggest you decide this way:
If you are trying to put together a good music system with audiophile speakers, strongly consider the Denon unit.
If you are putting together a system for TV, Movies, Gameing, etc., where pure audiophile music is not the main focus - consider the Onkyo unit.