I like doubles the best. It's just easier to get experience and I have fun toying around with different strategies. Gives you more to think about, and I find it enjoyable. I do love a good single battle though, don't get me wrong.
Ever since RBGY I've always wondered why trainers don't just send all six monsters out at once in a huge clusterf... hmm, we can't swear here.
Consequently I'm fond of double battles, never done a tag battle but the tag concept applied to any game/sport never really endeared me anyway.