Like master said in the beginning, there's not 1 certain gun that is best for hunting squirrels, it's mostly what you're in the mood for & your paticular style of enjoyment...
Personally i'd grab the ol marlin 60 (22LR) anytime before i'd use a shotgun for the simple fact i love to snipe them out of trees!!
Also My best bud that i hunt with always have a bet going on when we hunt together, & a nice clean headshot with a 22 is an extra buck not to mention a very nice challenge when the squirrel is at the top of a very tall tree or 100+ yards away!!!!
Definatly makes for great conversation afterwards...

Happy Hunting,
Mark