British English most of the time, although it does vary. I usually stick with 'favourite' and 'colour', but I will vary between 'centre' and 'center', because I don't think I was taught any specific way of spelling the word, so both of them come naturally to me. I taught myself to use s in 'realise' and other similar words when I realised that was the British way, but before then I'd use a z just because when speaking the word it sounds like a z phonetically.
But usually I use British. It annoys me when Americans tell me I'm spelling something incorrectly, when it's actually correct, so I do make a conscious effort to try and write entirely in British English. I'm just that pretentious.
All languages change over time though, and English itself is largely derived from other languages, so I don't really see a problem with it being changed. Although I don't see the need for it, what made some American suddenly decide it would be cool to remove the u from colour? It just seems pointless to have "American English" and "British English" it should be just "English" and everyone should speak and write in the same manner for convenience. Although apparently it's a better idea to change it...