This question is bound to open up a can of worms, but I'll bite anyway.
I think the best CHOICE (not actual best chip) is the Xecuter 2.3b lite or 2.3b Lite +. The lite is a solder chip, the lite+ is solderless. Both are pretty simple to install, with the + being simpler obviously. They are well known, work great, and have 2 banks for bioses, although so do many other chips.
Basically, this is really just opinion. I've heard good things about almost every Xbox chip, and very few problems except for a few non-working chips/user mistakes/etc.