You want to talk about reinforcing transports or about AA, because these are two completely separate things.
The M15 better fits into the Anti-Infantry / Armoured Car role than the Anti-Aircraft role, which I have never ever seen anybody use.
The M5 Half-Track is the best in the game, as it should be considering how late it comes. It's the lightest vehicle available by any faction at that point, but is the second most durable behind the Puma. The AA Mount is worthless at AA but great against Ostheer Infantry. There's also the Barton Special, which is far superior to Ostheer's Flamethrower Half-Track.
Second best is the Sd.Kfz. 251 Half-Track, this vehicle comes earlier than the others and is lightly armoured, even less than OKW 251's. It's most often used to keep up field presence by reinforcing, but is sometimes loaded up with Infantry and used to flank.
Last and unfortunately the least, the M3 Half-Track. Despite being identical to the M5 save for less armour, this vehicle actually has the durability of a T1 Scout Car. It cannot be used for anything but reinforcing, but USF already has the Ambulance for this purpose.