How do you go about flavoring buttercream for cupcakes? I want to make marshmallow buttercream, peanut butter buttercream, mint buttercream, raspberry buttercream and much more. I know for some buttercreams you can just exchange the vanilla extract for the flavor you want but is there other ways to do it?

you need to just add the flavors of your choice...

like for my raspberry i would use the juice from a thawed bag of frozen raspberries and that would be the liquid part of my icing,

Peanut butter just add the amount to taste to your buttercream receipe,

Marshmellow are you looking to use marshmellow fluff or melted down marshmelllows? mint you can use a mint extract
you can purchase many different flavoring oils or extracts aswell.....

Is there a recipe people like to use for cupcake buttercream? What kind of buttercream do you guys like to use? French, Italian, Swiss etc. Any better than others in your experience?

you can also get extracts and oils to flavor, such as Lorann Oils. Then can get into all kinds of fun flavors.

I use IndyDebi's a LOT -- yummy and very easy.
http://cakecentral.com/recipes/6992/indydebis-crisco-based-buttercream-icing

SugarShack's is good too:
http://cakecentral.com/recipes/5163/sugarshacks-icing-and-tips

and can even just put fondant on top (and have fun with all kinds of designs, embossing, etc.)

Search the recipe section under "FROSTING" there are tons of flavored buttercreams.