In terms of the number of Rishonim and Acharonim printed, along with the Rif and additional modern clarifying mevuarot, what is the most comprehensive גמרא available today?
I'm referring to those with the standard tzuras hadaf.
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The answer to your question really depends on what you mean. The gemara with the most modern explanation is Mesivta by Oz v'Hadar. Each volume covers approx. twenty folio pages of Gemara. A full set is humongous. Second would be Artscroll.
Neither has commentaries such as Rif though. If you want a gemara with standard commentaries in the back, it almost certainly would not come with modern explanations. As far as most Rishonim and Acharonim, most people would probably say Oz v'Hadar (specifically the red version of their Gemaros used in yeshivos.)
There is a version of Maseches Sukkah that is a one-volume Gemara that also contains the major mefarshim and poskim on Sukkah, but as far as I know it only exists on that one Masechta.
If digital does it, then nothing beats מפרשי האוצר mentioned above. The biu responsa project is also worth mentioning even if it does not near the mind blowing amount included in מפרשי האוצר.
For number of earlier works printed in one volume, it varies by masechta. Oz Vehadar 'red', the moznayim/ש"ס נהרדעא and וילנא החדש will all have a common basis from what was printed in the vilna shas (if reformatted)- רא"ש, רי"ף ומפרשיו, מהרש"א מהר"ם מחרש"ל, ילקוט מפרשים רש"ש, מרדכי, תוספתא.
They will differ by masechta within each series what is included. eg different collections of תוספות, the הלכות לולב of various rishonim. The oz vehadar will add חידושי הב"ח when available and additional notes on מהרש"א and רי"ף/ר"ן, and the vilna hachadash might add material from achronim like רע"א or the קצות . The נהרדעא will add ליקוטו תוספות on the page, and the oz vehadar will add רש"י מכת"י when relevant. I think they all have ליקוטי רש"י. On some masechtos you can find a kovetz that prints the gemara in the front, in which case you have far more printed in one volume (although that likely wouldn't include the regular ריף ראש etc)
Insofar as modern works rather than collections of earlier works, if you want the greatest amount of explanatory material in a single volume then Oz vehadar's Mesivta series has little competition. Machon Hamaor's Shas Lublin has more material on agadah if that is very significant.