1) ROTJ Sidious. While he has not touched a saber in the 19 years since ROTS, he has studied the force in all of its aspects, gathered sources from all around the galaxy. Basically check Gideon's quote for precision.
2) ROTS for basically the same reasons. ROTJ Sidious hasn't touched a lightsaber in 19 years. Granted, ROTS hasn't touched his in 13, but he seemed more than able to fight Yoda.
3). Would probably go to ROTJ Sidious do to his superiority in the force.
I think if it was just a lightsaber duel that ROTS Sidious would win because he is younger and is more agile. If it was a battle with Force powers, ROTJ Sidious would win without a doubt. And I don't think I have to explain why.
__________________ Master Fisto, may the Force be with you -- Yoda
Out of curiosity, is there any actual confirmed source stating that Sidious became LESS talented with a lightsaber in the 23 years between ruling a Jedi-watched democracy and ruling a Jedi-less empire?
__________________ Recently Produced and Distributed Young but High-Ranking Political Figure of Royal Ancestry within the Modern American Town Affectionately Referred To as Bel-Air.
Sidious outright states to Vader in RoDV that the only reason the Sith use a lightsaber is to "humiliate the Jedi." While there is no concrete evidence saying that he didn't keep up lightsaber practice, there is a lot to suggest that he didn't.
__________________ Introduce a little government. Upset the established gangs, and everything becomes order...
Democracy is the very definition of awesome.
__________________ Recently Produced and Distributed Young but High-Ranking Political Figure of Royal Ancestry within the Modern American Town Affectionately Referred To as Bel-Air.
palpatine cut down three master jedi in seconds without blinking...granted he's honed his force lightning in RotJ but a lightsaber can easily defend against lightning as we've seen many a time...
honestly, its going to be close, but in his youth he'd cut his older self down...
His older self has had 23 years of unhindered delving into all the galaxy's Force-strongholds. His Force-powers are greater than they were in RoTS. So if RoTS Sidious' Saber can't withstand RoTJ Sidious' Lightning...
__________________ Recently Produced and Distributed Young but High-Ranking Political Figure of Royal Ancestry within the Modern American Town Affectionately Referred To as Bel-Air.