Which of these modes can work at the lowest signal-to-noise ratio as measured in a 2500 Hz bandwidth?

FT8 is a highly specialized digital mode designed by Joe Taylor (K1JT) specifically for weak-signal communication. It uses advanced digital signal processing and Forward Error Correction (FEC) to decode signals that are deeply buried in noise, functioning at signal-to-noise ratios (SNR) as low as -20 to -24 dB. Before the advent of modern computer-based digital modes, Continuous Wave (CW) was considered the ultimate weak-signal mode. A skilled human ear can pick out a CW signal slightly below the noise floor, but it cannot match the extreme weak-signal decoding capabilities of modern computer algorithms. PSK31 and RTTY are also digital modes, but they lack the heavy error correction and specific weak-signal design of FT8, requiring a much stronger signal to be readable.