It really depends on what you want to hear...there's the power metal/prog mix Flight Of The Migrator, Pink Floyd-ish The Dream Sequencer, my personal favourite Into The Electric Castle which is just about everything...the rest I haven't heard yet.
You might perhaps want to get Star One - Space Metal (another Arjen-project). A very rocking progpower metal album so to say. That's the one I got first on cd, before ItEC.
I'd say The Dream Sequencer takes a while to get into at least, I didn't really find anything to grab in it at first. It just clicked one day and I've loved it since.
People often consider Into The Electric Castle as cheesy. Partly that might be true, but it hasn't stopped me from loving the album. Great instrumental passages especially.
Of course you should get The Human Equation when it comes out. Sounds like it's going to be huge, a little alike to Into The Electric Castle, but obviously more varied now that Arjen had a much better range of singers to use and hopefully grew as a songwriter too.
Some songs I'd recommend:
Ayreon - Amazing Flight, The Garden Of Emotions, The Two Gates, My House On Mars, Dawn Of A Million Souls
Star One - Set Your Controls, High Moon, The Eye Of Ra, Starchild