I don't know if there's any such thing as "the best" drum machine. They tend not to make crappy ones that sound like tin cans anymore, although some people like that. If I had a jillion dollars, I would get several, because they all have different samples.
I have the Alesis SR-16 and it does sound pretty realistic. Good for rock or funk. But for dance music it's no 909. It has a very scant number of cymbals and hi-hats, none of which really cut it for electronic music. That's the one big drawback, other than not being able to copy one drumset to another. It does have electronic/rap sounding bass drum and snare sounds, and very high tinny closed hi-hats. So you CAN do beats for electronic music, but not with much variety. It's mostly useful for wanting to sound like you actually have a drummer. It's not so much a drum machine as a "fake drummer."