One with a longer cable, nice thick cable on it. Wrap it throug the front removable wheel. The back, since it has the gears, is more difficult to remove. But if your cable is long enough, wrap it through there as well. Take the seat with you. Seriously. Unless your local bike shops have something to attach to it so you can securely lock it up as well. Thieves will look at the easiest things to remove. But if you're going to take the seat with you, don't put a rear mud guard on that same location.
On my bike, that was the only place... so guess what they took? the piece that locks my seat for riding and the d*** mud flap right under it. I had my seat in my apartment. The front wheel was locked up by the cable.
Basically, anything easily removable is a target. Portland is a HUGE biking city, and bike part thefts are a very ugly side of this city... very ugly and quite major. I can't afford to get mine fixed, so it's collecting cobwebs in the now finished 'secure' bike room in my apartment complex. I haven't checked on it in a while... I wonder if it's still there. Those parts were stolen right outside my front door...
a minimum 6 foot cable lock, if not longer... if you can't find one longer, go to Home Depot (or something like it) and have them cut you a length of coated steel cable and do the loops at each end... then you can put your own lock on it... a nice heavy duty lock. I did that a long time ago when I was in college.