It depends of the Episode:
ANH: colors of 1997 are better; Greedo’s death is better in the 2011 version; Jabba is slighty better in the 2004/2011 versions; no fucking cgi rocks in 1997/2004; better Ben’s dragon call in 1997; hum… hard call on this one !

TESB: 2011’s version any day. 2004 is almost identical but a few more things have been fixed (wampa’s arm, relfections in Bespin’s windows, sabers). 1997 has a best Emperor dialogue scene but 2004/2011 fixes continuity issue of how the Emperor looks; 1997 also has the terrible Luke’s scream; so yeah, 2011 is overall the best version. I don’t mind the colors of this one so I would even dare to say it’s my prefered version of them all, although I prefer Boba’s voice and a few dialogues of the original theatrical cut.

ROTJ: hard call again. I like the complete celebration scene of 2004/2011; don’t like Vader’s “No” of 2011; don’t like the sabers of 2004; don’t mind the colors of 2004/2011; like the “Emperor’s slug” thing fixed in 2011; I don’t mind (and I’m even used now) Sebulba passing by and the blinking ewoks; Shaw’s old Anakin beats Hayden’s young version (if George had tried to make a old Hayden’s Anakin make-up I would probably prefered that, be he didn’t); so it’s really close between 1997 and 2011.