To be honest, there's a lot of really fking good maps for CSS now.
Favourites for me...
Dust2
Dust (it's beautiful with HDR)
Prodigy
Nuke
Office
Aztec (has a lot more fans on source than 1.6)
Train (as an optional extra, but you need to adjust to it's play style)
If you want a hyper populated server, stick GunGame on it with smaller custom maps. It'll take about 2 weeks to get it busy and then itll be maxxed out every day from 8am to 12am, but there's no sense of community on gungame servers as the gameplay is just too damn fast for people to get talking...
So, yeah, I go with the 6 maps I suggested up top. Yeah, definately a 3min map limit though: I hate those ultra boring 1v1 campfests for 2 mins at the end of some games...
But, if you're looking to make it popular - Just use this link to pick the maps
http://www.game-monitor.com/topmaps.php?game=cstrike2