I had a rather difficult time choosing for this one. I was torn between Enid Blyton's Malory Towers (a series that helped me deal with a lot of issues while growing up) and the Harry Potter books (impossible not to like them!).
But in the end, I think the Asterix series wins hands down. It is guaranteed to make me laugh- anytime, anywhere. And not just laugh in my head, laugh out loud. Yes, my Latin is weak, and the classical/popular references sometimes go right over my head, but come on - who doesn't love Asterix, Obelix, Getafix, Cacophonix, and the rest of the crazy Gauls? This is one of those rare series that has timeless charm, for the beautiful illustrations and the brilliant humour, and more so for the sheer feel-good factor.
P.S. - Calvin and Hobbes comes a close second for this part of the tag.