I think Emperor in ROTJ is one of the best written and portrayed character in the series.
In ROTS McDiarmid over-exaggerated with the performance. But it might not be his fault. he might have been instructed by Lucas to act like that.
As for ESB, the guy looks like some boring grandpa. The actor even read the line like one. Still I prefer the old version to the one in 2004 SE simply because they made McDiarmid have the ROTS Palpatine make-up instead of ROTJ one (the two are actually very different). And I don't like prequel parasites in OT.