In my perception, if we are looking for solution which is flexible
and has versatile usability not dependent on any fixed structure in
the environment, then most simple locks do enough to protect the bike
from just being pushed away by someone.
So for a basic level of protection, a simple lock which can lock the
bike (the wheels with the body or to some fixed structure around) is
good enough.
Now to protect the bike from being picked up n carried away by
someone, a more technology dependent solution maybe better.
Like one of the members suggested that a device with GPS tracking
would be a good solution but the cost is high.
I suggest similar solutions which are more cost effective and easier
to implement.
One solution would be similar to the security system installed in
cars which starts sounding an alarm if someone tries to mess with the
car. The unit suggested by me for bicycles can be made using an
accelerometer or a gyroscope or a combination of both in a small
compact circuit and mounted on a waterproof device securely attached
with the bicycle. This would detect any motion of the bike after it
has been locked. If there is any motion on the bike caused by an
attempt to steal the bike, the device would detect it and transmit a
warning signal to a small key chain receiver kept with the owner.
The owner can check up on the bike if any such warning signal is
received.
Additionally the device can sound an alarm on the bike to startle the
thief and make general people around aware of the theft happening.
Additionally the device on the bike which transmits a warning signal
on being moved can be used to track down the bike in the same way as
wildlife conservationists track animals tagged with wireless
transmitters.
This device can be powered by battery which can be charged by wall
point chargers or by a small dynamo on the bike itself.
Once this device is activated, any attempt to steal or mess with the
bike is almost impossible to be undetected.
The device can be very affordable and compact.