I would always go for a D lock personally. Lugging them around in backpacks isn't fun(isn't great with any type of lock) if your bike will allow it then bungee cables around/over lock and pannier/battery/ect or any sort of holder so the bike takes the weight of the lock and not you. Front basket you can throw stuff in is another option.
As for picking the actual lock then it all depends on what you plan to lock your bike too and how long for, the best way to lock it would be to fold it in half and lock both wheels and part of the frame to something but then you are looking at a bigger lock.
Average size d lock VVV
http://www.amazon.co.uk/Kryptonite-Kryptolok-STD-foot-cable/dp/B00802R8Z0/ref=sr_1_7?s=sports&ie=UTF8&qid=1394574808&sr=1-7&keywords=Kryptonite
You can also go for smaller "wearable" locks, a couple of the better ones are the kryptonite mini u's with a holster(the belt strap thing under) or hiplock have one with some clip bits sticking out(or a chain you can wear like a belt).
Kryptonite mini U(the above one) VVV
http://www.amazon.co.uk/Kryptonite-Evolution-Kryptoflex-Cable-Bracket/dp/B005YPK9VQ/ref=sr_1_2?s=sports&ie=UTF8&qid=1394574379&sr=1-2&keywords=Kryptonite
I'm a bit of a kryptonite fanboy but abus, oxford, knog, hiplock, onguard, they all have some good locks out.