It's just too tough to compare the straight pool era to the nine ball era in pool.
In the straight pool era, the top five are surely Mosconi, Greenleaf, Crane, Sigel and Mizerak, but nearly all the straight poolers of that era were American. Of course, Luther Lassiter and Harold Worst may have been better than all of them if all-around play is the measure.
The nine ball era is much harder to gauge, but however you look at it, Shane's name is in the mix for greatest ever. There were two especially noteworthy crossover stars form the straight pool era in Sigel and Varner, who continued to be world beaters even when the primary pro game was changed. Each is in the conversation for greatest ever American player.
Shane's excellence at the US Open 9-ball and the Derby City Classic evidence just how great he has been on home soil. He has not fared nearly as well overseas, and in the large field, WPA sanctioned events offering the most elite fields in our sport's history (World 9-ball, China Open 9-ball, US Open 9-ball, All Japan 9-ball), he has never won an overseas title. That said, however, he has won silver at the World championships, bronze at the China Open. and bronze at the All-Japan, so he has, at times, excelled overseas even though the biggest titles have eluded him.
His name deserves to be alongside all-time legends like Sigel, Varner, and Strickland, Reyes and Pagulayan, but where he fits it the hierarchy of the nine ball era is not so easy to gauge. I suspect that if I made this list on some other day it might look different, but my hierarchy for this era of pool is:
Shane Van Boening
Throw out all the qualitative stuff. Greatness is measured in titles. Always has been, always will be. Shane's resume of titles validates that he's one of the all time greats.
I pretty much agree with the players you list above, although the OP was asking only about the American greats. Typically, in the modern era of Pool (post Hustler 1960) there have been one or two dominant American players during each time frame. In the 60's that would have been Lassiter in 9-Ball and Straight Pool, Eddie Taylor in Banks and Taylor, Boston Shorty and Ronnie Allen in One Pocket. Harold Worst was like a shooting star who went right to the top and then was gone just as quickly (regretably!). Ed Kelly was considered the best All Around player of that era. When 9-Ball became the dominant tournament game in the 1970's and beyond, Buddy Hall began his hierarchy, along with the emerging Mike Sigel. Steve Mizerak was the king of Straight Pool and Ronnie still dominated One Pocket. In the 1980's, Buddy had to share the top dog honors with Sigel, Varner, Mizerak and Strickland. Buddy and Strickland held court into the 90's, as Sigel began to bow out and Johnny Archer emerged as the man to beat. Varner was right there too, winning his share. The 80's and 90's were a very competitive era in American pro pool, with many other good players (Rempe, Davenport, Howard and Hopkins) capable of sometimes knocking off the big guns. The early 2000's saw the slow drop off in Archer and Buddy's game, as Earl and Varner continued their winning ways. Shane came along in the 2010's to change everything.
How I rate all these guys overall for their respective dominance.
1. Harold Worst - Brief though it was. Probably the best poolplayer I ever saw. He had no weakness anywhere or at any time.
2. Luther Lassiter - He really dominated 9-Ball and Straight Pool at a time when there were many great players.
3. Buddy Hall - For his longevity on top.
4. Earl Strickland - Absolutely the best tournament 9-Ball player ever! A speed above the rest.
5. Mike Sigel - He knew how to win, over and over again. Unbeatable if he got to the Finals.
6. Steve Mizerak - He was not just a Straight Pool player! He was at the very top in 9-Ball and One Pocket as well.
7. Nick Varner - He got the most out of his game, clawing his way to the top echelon.
8. Johnny Archer - Our best 9-Ball player, along with Earl and Buddy for a long time.
Where does Shane rank along with these great players. Right now I would put him somewhere in the middle of the pack. Unfortunately he has dominated American pool when the overall level of his competition is much lower than what these guys faced. Unlike the players I have listed, Shane has had to cut his teeth on a greatly increased level of competition that comes from abroad. Sigel, Varner, Earl and Archer fared well against a lesser field of international competitors. Basically it was only the Filipinos (Efren, Parica, Luat et al) that gave them trouble. The Euros were only starting to make waves in the 90's and the Chinese came along a decade later.
This is my first take on this and I may adjust it accordingly as I think about it more. All eight players that I listed above were among the greatest of their respective eras. That is without question. Buddy emerged in the 1970's and stayed at or near the top until the 2000's. For over 30 years he was at or near the top of his field. Varner and Earl are second to him in longevity. Lassiter also enjoyed a long illustrious career, winning tournaments into his 50's.