Well, since they're one of my favorite bands I thought I would give some input here. 'Out of Myself' is the first album I heard from these guys and I still think it's the best place to start for those new to the band. It will give you an idea of what Riverside is all about and has some of my favorite tracks such as "The Curtain Falls" on it. 'Second Life Syndrome' comes next, and is easily my favorite offering from these guys. Heavier than the predecessor, more varied, and with a myriad of standout tracks such as "Volte-Face", "Dance With the Shadow", and "Second Life Syndrome". The latest outing, 'Rapid Eye Movement', is somewhere on par with 'Out of Myself' in my opinion, but maybe not quite at the same level. It is however an excellent album and shows the more psychedelic tendencies of the band. My favorites on this one are "Parasomnia", Schizophrenic Prayer", and "Cybernetic Pillow".
You really can't go wrong with any of these three albums though, just an amazing band, through and through.