Which of these digital modes has the highest data throughput under clear communication conditions?
Why PACTOR IV is an adaptive ARQ mode that steps up to high-order modulation with data compression, reaching several thousand bits per second on a clear channel, far more than any of the other modes listed. The PACTOR family was designed specifically for message and file transfer, so each generation (I through IV) raised the top speed. The other choices are keyboard-to-keyboard or weak-signal modes whose speeds are fixed and very low by comparison.
Watch out FT8 tempts people because it is modern and popular, but it is a weak-signal mode that sends only about 75 bits in a 15-second transmission, making it one of the slowest modes in use; 45 baud RTTY and MFSK16 are both roughly 60 words per minute.
PACTOR I to IV: the number is the speed rank. IV is the throughput champ; FT8 trades speed for sensitivity.