The ULTIMATE mod chip will be the one that hasn't arrived yet and which avoids having to swap your backup in after a boot disk. The two candidates are NEO 4 (http://www.playstationmods.com) and MESSIA (http://www.channeltechnology.com/).

The most widely used modcip (so far as I know) is NEO 2.x (playstationmods). Also there is http://www.modchip.com, which is the modchip I use; not necessarily the best.