i still think Down is the perfect opening song. It's crazy energetic, one of best STP live songs, and the fact the first lyrics are "Pleased to meet you, nice to know me" is perfect for the opening song in the set
I think Dead and Bloated would be a great opener. Down would be awesome as well. I also think No Way Out with its sort of mellow start before it hits heavy would be great, though I guess its not necessarily considered a greatest hit kind of song. What if the opener had to be from the new album? I think the only song with a strong enough rock opening is Hazy Daze.