Croissants also differ as to location, and type. Also from one year to the next, changes in a bakery can lead to unwelcome changes. Mom and Pop for me is usually a good bet but not necessarily if there is only one bakery in a town!
Villandry in London used to do some excellent croissants and some other of the baked goods were really nice. I'm embarrassed to say I'm usually very satisfied in Italy with an Autogrill marmellata (croissant with apricot jam inside - a very mass-produced item) either in their cafe at the Duomo in Milan or at many of the Autogrill motorway service stations! provided it comes with a nice espresso... of course

In Switzerland I go for Laugen-croissants, made with a salt dough. Yum!
My best croissants of all time are probably the almondy nut croissants (they have a name that I've forgotten) at the original Laduree in the rue Royale in Paris. In the old days before the family that also owns Paul took it over, it used to be my "local". I recall seeing more than one famous French actress there regularly both before and after their various facelifts!
I've heard there are some very nice croissants in California, especially in the Bay Area. But so far I never managed to find award availability on the Air New Zealand flight I wanted to get there...
On a slightly different topic I'm usually very happy with the brioche at Pain Quotidien (and lots else at Pain Quotidien

).
ps I forgot. Some London offices I know are near some of the city centre branches of Tesco which sell fresh hot croissants in the bakery section. Get them while they;re hot... the Tesco's in Regent Street south of Piccadilly Circus and the one at Hammersmith Underground station are a couple of those that do this. Mostly in the mornings there are several hot deliveries. Price-performance is pretty good on these.