Generally Nintendo's games, because I keep returning to them and they usually feel they're the most 'fun'. Pure, innocent fun and clever gameplay. They also have a couple of my favorite franchises and favorite games of all time.
That definitely doesn't mean I'm exclusive though. I play a lot on PC as well (however I only play typical PC genres on PC) and on a PlayStation usually, but that depends on what console gives me the best value and games for my tastes. In fact, in Smeag's Top 50 Thread last month, I have more non-Nintendo games than Nintendo games and PC wins as most featured platform, though the Top 10 is dominated by Nintendo games. Also, neither my #2 favorite console nor my #3 is a Nintendo console.
Still, like I said, Nintendo's are the ones I'll go back to, and the ones I've been buying the longest and the ones where I buy the most sequels. I've been familiar with their games ever since late 3rd gen/early 4th gen and they don't get stale or repetitive. Having said that, should they ever stop making consoles, I'd just go wherever Zelda goes .