As far as fans here in Britain go, a lot has to do with what the teams were like in the 80s when the game became popular here, and which towns have major universities that attract international students.
The pecking order is
Bears (the whole Fridge thing, great schools, plus Brits like nasty defences)
Dolphins (Marino was the darling of the country exactly because he never won it all - we like a lovable underdog here)
Giants (NYC, vacations, good schools, nasty defence), Steelers (nasty defence, more good schools), Niners (80s glory, gorgeous holiday destination, even more great schools, latterly nasty defence), Ravens (very nasty defence).
Patriots get some love, some through bandwagoneering, but also because of the dedicated missionary work by Boston expats. You know how obnoxiously dedicated Boston fans are with their teams. Also, lots of great schools up there.
I guess I'm typical here: jumped on the SF bandwagon as a lad in 1988 (ring a bell? ), was in grad school in Philly when we won our last SB (so now I'm a Phillies/49ers fan, weird huh?), and I come to SF for hols every year without fail (well, it is absolutely beautiful).
[ Edited by paulk205 on May 27, 2012 at 6:48 AM ]