Cetaka has a really good point when he brings up both players, because they truly are in their own leagues. Put Matt against Chris in some prevalent tournament-style charts (charts that aren't purely intended on speed/simple patterning/etc) and Matt will lose a very large majority of matches because he doesn't care too much about things like crossovers/jump-usage/whatever -- he cares only about speed.
Conversely, put Chris against Matt on any sort of speed-oriented file (and when I mention speed, I'll just use the Tachyon pack standard of 15 NPS+, or 225+ BPM 16ths), and Chris will lose a good majority of files because he simply can't sustain it as well as Matt can. Chris is a great player and has shown that he can FA 16-block charts, but Matt is phenomenal at sustaining accuracy even when tired.
They are both great at what they do in different parts of the game. In fact, Mad Matt used to be the best PRIMARILY no-bar player (primarily meaning main preference of play), but
Prolix is easily second by far, because I can't think of any other ITG players that can stream that speed OR pass 17s no bar (other than Matt).
Then there are people who are excellent at double, people who have absurdly consistent accuracy, people who are excellent at mod-reading without any sort of memorization, etc. ITG is so divided now, but the community seems to be sustaining different styles of play now - at least, tournaments are now in support of running double-related events, and the big Midwest tournament (University of Illinois Champaign-Urbana) runs singles, sightreading/mods, double, and couples, which really promotes all sorts of play.
With that being said, I personally find it hard to say who is the best ITG player, because best can pertain to a number of different (relevant) things.