For the vast majority of smokers, it’s hotter at the bottom, and hotter at the back.
That means:
The hottest part is the back half of the bottom rack
The coolest part is the front half of the top rack
And if that isn’t the case for yours, it’s more often than not just a lot more even throughout, but still hotter towards the bottom (as the hot air and smoke rises from the bottom and cools as it travels further upwards).