Originally posted by Advent
But, I do recall there being at least a couple of more quotes that do specifically call him the most powerful Sith Lord that we know about. Besides, it's hard to ignore the tons of evidence that indicates he should be; Palpatine is basically the most profiled Sith Lord when it comes to his knowledge and power.
There is a downside to that level of exposure however; while his strengths are profiled so are his weaknesses. Specifically, the movies (highest form of canon and as such set the precedence) quite effectively establish limitations to both the likes of Sidious and Yoda; Yoda struggles with relatively basic telekinesis frequently throughout the movies, and at the same time was able to both overpower Sidious directly with telekinesis and later force a stalemate with the Sith Lord.
Even outside of the movies he is never shown to have done anything of a truly significant scale under his own power with defined circumstances.
The Force Storm (which is performed by a later incarnation anyway) relies on a complex and organised, indirect procedure that summons power from a "hidden reservoir of dark side power" (and as such is not reflective of his regular level of ability).
Palpatine's part in diminishing the Jedi's sensitivity of the Force was done in completely undefined circumstances, and it's entirely possible he required the use of tools or an organised ritual, such as the one we see in Sithisis, in which case it would not be reflective of his regular level of ability either.
Everything else IIRC has all been of a completely unspectacular scale, and nobody has ever really substantiated what knowledge actually means in a combat situation. The Force hasn't truly shown itself to be manifested through combat relevant techniques in that many ways; Jedi and Sith are shown to be able to use physical, long ranged offensive powers, both externally and internally on an opponent, defencive powers, can effect their own physical abilities, can hide themselves in the Force, sense hidden objects in the Force, enhance their reflexes, use telepathy both offensively and defensively and a few other relatively basic abilities (and rare ones such as Shatterpoint); Sidious himself hasn't displayed anything combat relevant outside of the above mentioned abilities so against someone like Bane (who himself has a pretty incredible knowledge base, and has specifically displayed all of the above), the only advantage Sidious can truly be said to possess is a variety of techniques; nothing indicates that he would possess any real advantage over Bane (such as an attack that Bane doesn't have a defencive ability for), knowledge wise; whereas everything Bane has displayed would suggest that he'd have a pretty considerable advantage over the Sith Lord in both power (absorbing, storing, defending himself internally against, and telekinetically redirecting a planetary level of Force lightning) and precision (subatomic scale alterations using the Force), not even mentioning the orbalisks.