S-Q-L without question.

* It works in all contexts ("sequel" gets clunky with names like PostgreSQL)

* It's not some aircraft company's trademark

* MySQL officially states their product's name is My S-Q-L (though they don't mind MySequel, but it's not the real name)

* Unlike "ARP" or "BIOS", "SQL" doesn't map to just one pronunciation. "Squeal", "Squirrel", and others are just as "valid" as "Sequel". Frankly, if I had never heard of SQL/Sequel and you put those three letters in front of me and made me pronounce them, "Squeal" is probably what I'd go with.