What's the best diaper bag in market now? Preferably a backpack..

That will depends on your budget. If you are looking at below $40, you can consider this Japanese bagback. Seems like a fad now. Seen some mother carrying using the bigger size bag as their diaper bag. It is very spacious and light for carrying around. I got mine from Q10. It is having sales now. You can refer to attached link: http://www.qoo10.sg/gmkt.inc/Mobile/Goods/Goods.aspx?goodscode=414622953&stcode=77