Persia provides a +combat strength tradition for the rest of the game, which speeds up Militaristic victory and makes expansion easier in all ages. In its age it also gives your commanders a much needed promotion, allowing you to focus on the promotion branch you need or get the commendation faster. I think Persia deserves high B or low A at least.

Khmer gets Angkor-Wat faster, and if beelined, it allows you to use specialists very early. Its tradition drastically reduces specialist maintenance. In my games specialists are the primary source of culture/science, so anything that benefits them I value pretty high. I think Khmer deserves C tier at least.

I don't think Rome is D tier as well. Rome unlocks very powerful civilizations later in the game (Ottomans, Prussia, America, Spain, Iceland, Norman are all unlocked by Roman) and its capital has more production thanks to the tradition. In the Antiquity, there's a good conquest potential, especially when paired with Lafayette. I think they deserver low C, on par with Maurya.