Must see places in western Europe

H. FINDLAY, NORTH RYDE

That's a lot. Even with six weeks to play with, five major European countries, each a major destination in its own right, is a full plate. If you want to cram in as much as possible you're going to be spending a lot of time travelling from one place to another rather than experiencing what is to be seen.

European cities are packed with wonders and you need time to absorb them, otherwise you're going to be racing  through galleries, churches, streets, parks and museums and everything becomes a blur. My minimum recommended stay anywhere in Europe is three nights. That gives you two full days for exploring. If you believe a destination deserves less I question why you would want to go there in the first place. 

Hitting the tourist hot spots is fine but they're unbelievably crowded. One of the joys of travelling in Europe is getting away from the tourist hordes, and even in a place like Venice that's possible and rewarding, but it takes time. 

In six weeks I'd suggest you could dip into France, Switzerland and Italy but for all the reasons above leave Spain and Germany for another trip. That would give you enough time to take in Paris, Provence, the Cote d'Azur of southern France, the Italian Lakes, Interlaken and Lucerne and maybe one of the classic Swiss rail journeys, Venice, Florence, Tuscany and Rome.