Hm. As you're from Finland, how about LHR (as Gary is always banging on about)? It probably has the best mix of traffic. I would also endorse ZRH (although you'll see a lot of Swissair Airbuses), and AMS (though there are times when it seems every aircraft is a KLM 737, F100 or F50). The best place to go kinda depends on if there's particular airlines you would like to see, or if you are just interested in an airport with really good spotting places.
Gary, when are you going to MIA? Only I'm looking at going down there in Oct: we could make that an A.N "outing"!