All three Antimatter albums are excellent, and are definetly on par with each other in terms of greatness.
"Savior" resembles Portishead quite a bit more than the later albums... perhaps in part due to the resemblance in artwork, but it also carries a vibe similar to Anathema's "Alternative 4" to my ears. This album features female vocals almost exclusively, and has the most clinical production of the three albums with the most obvious programmed drums sound.
"Lights Out" is easily the darkest of the three, but perhaps the least consistent. "In Stone" and "The Art of a Soft Landing" are easily my favorite Antimatter tracks, and quite possibly THE most depressing songs I've EVER heard. The second half of the album features alot of filler though. Many of the songs, such as "All That You Know is Wrong" favor Pink Floyd over Portishead, although the dub elements have not completely been thrown away.
"Planetary Confinement" has the most organic sound of the three albums... being the only one to have live drums and focuses less on the keyboard elements; it makes me prominent use of acoustic guitar. The album alternates between Patterson's tracks which all feature female vocals, and Mick Moss's tracks featuring his own vocals... so it's almost as if the album is a split between two similar sounding, yet distinct enough on their own bands. As was mentioned earlier, "Line of Fire" is an amazingly dreary song... just thinking about it makes me want to go to sleep. This album is also the most difficult to draw comparisons to any other band with.
I don't really have a preference among the three albums; all are definetly worth checking out. I hope these descriptions will help you find the one which is most in-line with your tastes!