I believe a fair assessment of "accomplished" player should include the number of times you beat a top 10 in major tournaments, h2h against prominent players, and overall GS success in all surfaces. I put Zverev on my top 5, but he is a total of 0 wins out of his 39 top ten wins on bo5 matches. This speaks volume. Interestingly, he has a high GS PCT 66-26 (71.7%). He is tied 4-4 against ND on bo3 matches, but 0-3 on bo5 matches.
I'm higher on Nalbandian than many because his "A" game gave many problems. He is one of the few that gave prime Federer problems (8-11) and he also had positive H2H against #1 players as Moya and JC Ferrero. In addition, he got to QF or better 10 times divided between all FOUR slams. His GS record 86-36 (70.4%) is among the best for non-slam winners with 75 or more wins. Finally, his display at the Madrid Indoors when he defeated all the "big three" is still one of the greatest M1000 performances. Nadal was able to do a similar feat when he defeated Murray, Djokovic, and Federer on his way to Hamburg 2008. Not to mention had one of the greatest backhands in recent times.
Corretja won 17 titles and was collectively 5-2 against both Nadal and Fed (yes they were still green), and he was 3-3 against Hewitt. He was overall 61-38 in GS and made only 1 QF outside of RG. He did win a Bronze medal in Doubles and 31 top 10 wins. I thought he was a bit better than F. Gonzalez who won 11 titles and had 27 top 10 wins. FG really played well in the Olympics (10-2) where he won 3 medals (1 Silver, 1 Bronze, and 1 Gold Medal in Doubles.).
Ferrer made my list mainly due to his 27 titles and 54 top 10 wins. In addition, 145-63 GS (69.7%) but he was 1-6 M1000 finals. Davydenko, Corretja, and Zverev all have winning H2H against him. but he led Nalbandian 9-5.
Martin did make 2 GS finals and he has 34 wins top 10 (8 were bo5) while he enjoyed a 102-48 GS record. However, his total of 8 titles rate him slightly lower than some of the aforementioned.
Davydenko is 21-7 (75%) in all Finals and he did help Russia in winning a Davis Cup. He won 3 or more titles in 3 surfaces (HC, Clay, Carpet) and is known for having a 6-5 record against Nadal (yes I know 7 matches were played in hc) but he was a bit of a thorn to Nadal in Finals. However, Federer did own him. Davydenko had 38 top 10 wins. Furthermore, he is 3-0 in M1000 finals.
I think Zverev most likely will win a slam so this will eliminate him from the list. Expect some NextGen players like FAA, Shapo, Tsitsipas, and Rublev to battle it out for this dubious distinction.