It all depends, if you collect numbers & want quantity, then places such as Heathrow, Birmingham, Stansted, Gatwick & Luton would be (possibly) the top 5 'spotting' locations.
However, if you collect numbers & you want quality, then places like Coventry (various old props, Dc3, Dc6, Electra & Russian Freighters), Biggin Hill (some interesting GA visitors & based aircraft) & East Mids (some nice freighters) would go into the list.
Same with photography, if you want photos of B747s, then Heathrow would be No1, if you want pics of Electras, then you'd make Coventry your No1.
I personally like looking at all those shiny nice bizjets at Luton (we can all dream)
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