Let me start out by saying that I'm a map junkie.  I spent way more in maps and hauled more paper maps than any sane person should have.  For me, it is part of the fun... but if you're only going to buy a few, here are the ones you should buy.

 Overall, I'm a fan of the ITMB maps  from Canada. They happened to be the maps that Key Maps in Houston carried the most.  I had one for _every single country_ I would drive through.  MASSIVE overkill, especially in Central America.  The Waterproof ITMB maps are superiour and while they're hard to write on, they are the only maps that will take the abuse of being folded in bad ways in a tank bag.

The one exception is Mexico, where you should use the Guia Roji.

Here's my list of maps that I recommend if you're going to traveling the similiar overland route from Mexico to Argentina:

Mexico: Guia Roji Mini Atlas - Covers Mexico, Belize, and some of Guatemala

Guatemala: ITMB Guatemala - This one will cover Guatemala, Belize, El Salvador, and most of Honduras

Costa Rica: ITMB Costa Rica - CR, Nicaragua, and most of Panama that you can drive

Central America: ITMB Central America - This one is a must... You could get away without the Guatemala/CR maps if you just had this one.  Too bad they don't have a waterproof version.

For South America, because the countries are so much larger, I generally used each countries' own ITMB map.  Instead of providing links, you can just look those up yourself :).  The only exception is for Chile. You can just use the Argentina map for Chile unless you're going to North Chile.

The special case for South America is Bolivia.  I tried using ITMB, Nelles, and another German map company's map and none of them were reliable.  Often the maps would say that a road was paved and it wasn't.  In Uyuni, I even went around town to the little paper shops and bought a printout of a map that was supposed to be good, but even that was wrong.  My GPS maps were wrong too... Nothing was useful in Bolivia except for dead reckoning and asking bus drivers.