Curious to see what you all think. Personally, however, I think Spring, due to all the flowers in some biomes, the fact that most biomes' foliage isn't dried out/super lush, the fact that deciduous trees (oak, birch, dark oak) still have green leaves, and last but not least, the fact that only polar biomes and some mountaintops are naturally covered in snow and most areas (except desert, savanna, and mesa/badlands biomes) are very rainy.
It's hard to tell but I think the springtime. After the 1.7 updates with those new biomes added, I'd definitely say springtime. I've heard, many many years ago, of an idea to implement a realtime season engine in Minecraft; never happened... Such a shame... :\
Yeah, no real seasons for me as well, it simply heavily depends on your location. And although I think it could make Minecraft pretty interesting if they did introduce seasons I also can't help but wonder if it wouldn't just make things even more confusing (days which get shorter and longer for example). Still... If I had to pick one I'd also be tempted to pick Spring as the constant season.
Seeing how minecraft exists on a flat earth, with a sun that never changes its angle of rising, there can't be any changing seasons and temperature differences in biomes must be determined by something else, maybe geothermal heat.
