Obviously, PoS started with Peercoin. There was a reason I ended the sentence with "as it's accepted today". You should and I know that Peercoin's PoS is inferior to NXT's PoS implementation.
Inferior in which area? As I know Peercoin has less theoretical PoS security problems because it's using PoS and PoW simultaneously.
AFAIK Peercoin uses centralized checkpoints and opens up the possibility of stake grinding. But I'm not 100% certain.
In the end DPoS is probably one of the best PoS integrations, as it scales wonderful and allows the blocktime to be very fast/constant.