"the best one" really depends on what you want to do. "the best" vehicle for winning a race is a race car, not a tractor... but that race car won't do shit for your fields.
The flexNT is a high performance, longer range version of the xNT. They both use an NTAG216 chip inside, which is ISO14443A and NFC Type 2 compliant.
The flexDF is a high performance ISO14443A and NFC Type 4 implant with the DESFire EV1 8k chip inside. It can operate with multiple "applications" instantiated and has ~8k of total storage.
The flexDF is physically larger than the flexNT but it also has 8 times the storage space. However, the flexDF isn't necessarily compatible with the same devices the flexNT is. For example, both are ISO14443A and the entire line of Samsung Ezon door locks purports to support ISO14443A tags, however it will not read the flexDF, while it will read the flexNT and xNT just fine.