In my opinion the mechanized list is the strongest late war American list. It provides excellent mobility, protection, and firepower.
Your basic combat platoon in an American mechanized force is one of the most diverse units in the entire game. It has a lot of anti-tank and anti-infantry stopping power, plus when they are mounted they are virtually immune to small arms fire. It is also large, meaning it is hard for the enemy to force a moral check on them. Lastly, as a bonus, each unit has 50 cal-machine guns mounted on 2 of the 5 half tracks, and AA mg’s mounted on the other 3. So the unit has more than enough killing potential with out even disembarking, and the 50-cal’s can destroy opposing half tracks and light vehicles. I would always recommend taking as many of these guys as you can afford.
Your entire force cannot just consist of combat choices (unless your playing a very small game), so you will undoubtedly need to fill your list out with support options. A good place to start is to make up what the combat choice is missing, and that is down range Anti-Armor, Armored assaults, and artillery.
You have a few choices in the anti-armor category, unfortunately they are all mediocre. You could take 57mm guns, they are cheap and fairly reliable in dealing with medium armor, and they come with more 1/2 tracks. Unfortunately they have no HE shells and if they don’t start deployed it will take several turns before they can start firing. Your next choice would be the dedicated tank hunters in their M-10’s. They do have the highest AT in the American force, allowing them to easily destroy medium armor, but they suffer from some pretty bad special rules which make them very, very reliant on terrain heavy tables. Also, they are very costly and not very durable. Lastly, you could pick your self up some good ol’ Sherman tanks. These guys will be just as effective down range as the 57mm guns, but are also armored so they can stand up to anything short of dedicated Anti-armor guns. Plus, they can perform tank assaults when you need that extra push. They are not heavy tanks, but they hold up a lot better than M-10’s and don’t suffer their negative rules either. (I don’t care what people say, I find the American tank hunter rules to be too costly, situational, and more often than not, detrimental.) If you cant deal with enemy armor down field, your half tracks become mobile death traps.
The next gap in your list will be Artillery, and that can be easily filled with a few priests. Artillery is very necessary, you need to be able to pin or smoke out key enemy units to allow your half-tracks free movement on the board, and a small group of priests fill this role well. Obviously, your list is your own, but you would be a fool to not attempt to take at least one group of priests (minimum group size works best). If you can’t smoke out enemy AT guns, then your half tracks become mobile death traps, again. Also a key note about artillery use in a mechanized force, it is not about causing casualties; it is about keeping key units pinned or blind to allow you greater mobility on the board. Your combat units will have more than enough anti-infantry fire power to deal with people out in the open, but they wont get there if anti-tank guns blow your half tracks away.
One more unit that warrants some thought, the Stuart. This little tank is fast, and more importantly is just as effective in an assault as a full blown tank. They force tank terror, just like the big boys, and that can make a huge difference in an assault. So if you don’t want to spring for the Shermans, or you think they will be busy dueling tanks, then these little guys could come in handy. Also their guns are more than capable of punishing any medium tank that shows its side to them, and they are fast enough to flank. Not everyone will have the points available to spring for these guys, but they do warrant your consideration.
Don’t forget, you need not actually use half tracks if they are not to your advantage; for instance, if the opponent is using an armored list. Before the game starts decide whether or not your half tracks will help you or not. If your defending a point from an armored foe, then dig-in with your large platoons and force them to assault you plethora of bazookas, don’t give them easy half track kills to force your moral check sooner. Remember you can dismount your forces and send those half tracks to the rear at any point in the game (once they are gone, you cannot re-summon them). You can also dismount the guns on the half tracks by simply replacing a rifle base with an appropriate MG base (this lowers their save from a 3+ to a 5+, but gives you a lot more shots. Best used when your digging in around an objective.)
Lastly, remember to have fun, or else what is the point of playing?