>>2019567
Overall it's probably Bastions as >>2019789 says, but I want to give an honorable mention to Barbarian Warriors for being arguably the best cultural T1 unit in the game. Stun+automatic defense mode is great, and Savage Strike means they do above-average damage for a defensive unit. Optional cavalry is a nice boost as well if you can fit it into your build.

Clay Defender is cool but you kinda have to go for a full clay unit army to get the most out of them due to the Legion trait.