Which DaVinci processor is the best for low latency H.264 encoding?

Since you post in the DM3xx forum I am guessing you are looking for a DM3xx recommendation?

The DM365 actually just received a new version 2.0 h.264 codec with low latency support, the codecs themselves can be found here. It now allows for output at the NAL level of granularity for very low latencies at the codec, which should allow for <2ms of codec latency as mentioned here. Keep in mind that these codecs are still very new however, so full integration into the DVSDK stack is on going.