I'd say that at the beginning, it was a Roger thing, then Nadal came along, it became Roger-Nadal(with Nadal better in h2h, but Roger better overall), while Djoko and Murray were behind, but Djoko managed to emerge to rival the other two while Murray was still lagging behind, and although Wawrinka ended up winning 3GS, i'd not really put him on Murray's level, who was the closest to concurrence the big3 for a long period of time, won a gold medal and already has been number1
At one point, Del Potro had means to rival them, but injury struggles didn't allow him to. Then suddenly Djoko started gaining an ascendance over Nadal, then Roger especially after his knee injury and growing older, but Roger ironically started finding solutions to Nadal late in his career, with his battles with Djoko almost ways being close, ultimately then placing the big3 ahead of the rest, but me thinking tennistically, Roger is superior
Currently among the youngsters, the one who interest me is Kyrgios(although controversial) due to his natural talent. Just hope he calms down a little and become more tennis focused. There is also Thiem's game play i'm very interested in, although he isn't really in the youngster generation anymore, but i think he is the one who really grew the most thus far. Then there is obviously Zverev and Tsitsipas
Technically speaking though, at their best, the big3 destroyed multiple generations, so it'd be hard to find replacements to them after they'd retire