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I assume you mean which side offers the best views of the canal. Personally I don't think it makes any difference and during the 8-9 hour transit its best to watch from the open decks or, if your lucky from the helicopter pad. Even if there was a best side it would rather depend which way you were transits the canal - west to east or east to west.
I entirely agree with ms Adams, but just to be pedantic, the Canal actually goes North to South, or South to North!
Sarf East to Norf WEST and exit Norf or Enter Sarf transit Norf West to Sarf East truly pedantic :-)
Would suggest port side west bound and starboard eastwards but out on opendeck best and flit between.
It also depends which of the two channels into the locks you go - I've been through it twice and on the Gatun side have gone through the locks on the left hand and right hand channel, coming back though have only done the left hand channel. As said above you get a great view on an open deck and even from your balcony you'll get to see something!