Since I have only watched CSI: Miami, because of David Caruso, I would have to say that it would be a toss up between "Under Suspicion" (S4, Episode 6), "Rio" (S5, Episode 1) and "Ambush" (Season 6).
"Under Suspicion" was excellent because the team had a hard time dealing with Horatio as a suspect, but they handle it with such poise and professionalism.
"Rio" was beautiful because of the location in Brazil and how H and Eric went after Riaz on their own time. It just shows what lengths people will go to protect those they love.
"Ambush" because it was a followup to what happened in "Rio."
I could name a few bit more, like all of the season finales, but it would take up too much room!
As far as CSI LV...the ones I remember watching were the one where the decapitated head was rolling along highway, another where one of the CSIs was inventing his own CSI game. And, of course, the one where they paired up with CSI Miami.
And CSI: NY...of course, the ones where they pair up with CSI: Miami.
The CSIs shows need to pair up more often. It's fun to watch their different styles collide.
But, I will be watching as many CSI LV episodes over the summer, because I've heard excellent things about it. Then I can post which ones are my faves!