Won't say greatest but NWO appearing on RAW in their 1st match I think against Rock & Austin(Not sure) was one of those "5 years ago, If you said this would happen I would've laughed in your face" type moments...
There are alot of great moment's and it's impossible to name each one, but some that spring to mind are
Nexus's first attack on RAW
SCSA returning to RAW to save the WWF
When Justin Gabriel pinned super cena(yeah i'm a mark for JG lol)
just about anything that featured Eddie Guerrero and or the rock
Shane jumping off the titantron on big show(it was on raw i think)
and many more moments...
When the Radicalz (Chris Benoit, Dean Malenko, Eddie Guerrero and Perry Saturn) first showed up on raw and attacked Foley (i think).
DX Invasion of WCW (saw it on youtube and i regard it as a good moment)
The return of Sexual chocolate (Old School Raw)
The Rock concerts
Y2Js return to Save.Us