Let's stop for a moment from cherrypicking people at their hypothetical peaks or using gut feelings and look at the numbers. Here are the major players since 2000 who faced each of the Big 3 at least 5 times each and how well they performed against them + Next Gen, who haven't necessarily played each of the Big 3 5 times. The green highlights when a player actually has a winning record over one of the Big 3. Thiem has easily won the most against the Big 3 percentage-wise while Murray has by far the most total wins against the Big 3. Among non-slam winners, Zverev, Nalbandian, Tsonga, and Tsitsipas head up the list. But let's say, you feel that Zverev has only been playing the Big 3 at 80% of their prime, well, multiple 37.5% by 80% and you get an "adjusted" 30% that would place him between Hewitt and Tsonga in terms of success against the Big 3. But basically, Next Gen has been a bit underrated in this thread. Also, poor Gasquet.