What is the best way to pick off banelings with zerglings?

You can do something like pre-emptively assign a ling to hotkeys 9,0 and order them to follow another ling in your 1 group. Then when you want to pick off a baneling hit 9 and right click and order 1 away.

The only issue is if the ling the one in 9 is following dies, the 9 ling will stop following your pack around. Also, if you engage in combat and retreat, the following ling won't retreat until the ling its protecting is a significant distance away, which will probably get it killed. Against banelings this might be a good thing, however.

Similar to your #3, but less dangerous. Only works if you're being actively chased by the banelings or they're in aggro range:

Select your entire group. While holding shift, alternately click one zergling out of your control group (from the wireframes at the bottom, obviously) and right click an area to move on the map (new waypoint for the group). This will result in you "leaving behind" one ling at every waypoint. The uncontrolled single lings will aggro the banelings.

Note that if you leave your waypoints too close together your single lings will bunch up when aggroing and possibly get splash killed, so give them space. If you have speed you may be able to run circles around the banelings with this method, aggroing in a ling at a time.

I would personally divide my zerglings into two seperate hotkey groups at a ratio of about 8:1 or so. Then I would take the smaller hotkeyed group and attempt to do your solution #3. That way if you fail and accidentally sac your entire group rather than just the single ling, you've only lost about an eighth of your original force.