Good question, let's see.
Lufthansa comes to mind, I've been flying with them quite much, and they have invited me to the flight deck both in their A321 and B737-500, so there's no way I can leave them out. ;D
Northwest didn't inpress me much (sorry Craigy), I rode on their 744's over the altantic when I lived in the States, however their subsidiary Mesaba (Northwest Airlink) was a great experience. Nice crew, and a swedish aircraft (SAAB 340) ;D
KLM are cool, but much because of the aircrafts :
First ride on a Jumbo was with them, flying on a 777 which were about 3 weeks old and of course they operate 737's daily from my town to AMS Schiphol :)
And then we have SAS, the joint venture between Sweden Norway and Denmark. Pure quality, but they are on the total opposite scale of SWA for example. No funny crew whatsoever, This is an airline strictly for dull businessmen ::)
Have flown on every aircraft on their fleet except their A330/340 which they ordered to replace the 767's :P
So here's the deal.
1. Lufthansa
2. KLM
3. SAS
4. Mesaba
5. Scanair (defunct SAS charter flight airline, I flew on their DC10's to Mallorca every summer from 1991-1994)
