Well the reason I'm wondering where the other nukes could be is because in season 6 they've stated there have been attacks in St. Louis as well as Philadelphia. I don't think all the other nukes will be in the LA area, but who knows.
But they needed a "special guy" to program the nuke that went off with a device they acquired - I wonder if the other 4 nukes still require some sort of activation device?
Ranking the seasons might be fun... I've seen each season twice now, buying them on DVD when they go on sale is hard for me to resist.
I'd say here's my rankings of the seasons -
#1 - Season 4
#2 - Season 2
#3 - Season 5
#4 - Season 1
#5 - Season 3
But hell, they're all great!