The best time is subjective- I like going during the World Series of Poker because the poker action around town is awesome. Cash games are big every places, and the daily tournaments draw a lot of people. (330 at a Noon tourny at Caesars one day). I also like during the fall and spring- when it is nice enough to walk on the strip. and cools off a little at night.
The cheapest time is the week right before Christmas- and that was a good time to go as well. Also after July 4th through August is slower as well.
The most expensive- New Year's Eve- the rates are off the chart. Big weekends are also expensive (Super Bowl, The NBA All Star Game being in Vegas this year, the NASCAR race weekend, three day holiday weekends can draw them in as well)
I like going about any time- but the summer is very hot and that would be my least enjoyable time as you really can't walk the strip.