All of them have potential to be rockstars: they each got dedicated mentions in the Top 10 Gen 4 Pokemon guide. Comparing their best possible movesets with what's in the game, here's what we get compared to non-PB Groudon:
-------------------------------------------------- DPS ---------------- TDO
Groudon: Mud Shot/Earthquake ------- 13.867 ------------ 504.3
Rhypherior: Mud Slap/Drill Run -------- 13.641 ------------ 554.3
Mamoswine: Mud Slap/Earthquake --- 12.91 -------------- 362.1
Garchomp: Mud Slap/Dig ---------------- 13.635 ------------ 465.6
As seen, Rhypherior and Garchomp come neck and neck to Groudon - arguably, Rhypherior actually takes the ground type crown. This is entirely dependant on if Rhypherior is given Drill Run and Niantic doesn't default to throwing Earthquake on it like they do with every fully evolved ground type. If given Earthquake its DPS falls below Mamoswine, yes it still maintains higher TDO than Groudon.
This is all in a vacuum: Going up against an electric boss such as Raikou, Garchomp will be leagues ahead in the TDO department thanks to its dragon typing while sporting similar DPS, which actually doesn't vary much between Dig and Earthquake. Mamoswine falls short in the ground department, which may be for the best since it's better suited in its ice type role.
To answer the question asked, both Garchomp and Rhypherior stand to rival Groudon heavily, and in some cases may even usurp it given the right moves to do so.