I don't have a favorite game. I play whichever games are on the weekly menu. I usually eliminate the games from my personal, "eh, it's okay" list by trying to get a seemingly impossible (for me) badge.
A well-timed example: I used to really like it, but now Mahjong Garden gives me a headache. I'm just under three-quarters through (yet again) the Libra badge attempt. This is my 4th attempt. Yes, I've spent seven weeks on it so far. ::vomit smiley::
I did get sucked into the badge/glory thing. Five more years, give or take, and I'll have 'em all*. Hurrah.
*Weeklies only. I won't spend more than 13 diamonds (78-ish cents) on any one badge, so that knocks a lot of those out of range.