Probably top sirloin or round steak. I prefer top sirloin myself as it has very little marbeling and it's easy to trim any fat around the perimiter. London broil has more fat and is much tougher than sirloin however it's usually a good deal cheaper. I'd suggest marinating and/or tenderizing it before cooking since it tends to be tough.
If you go the top sirloin route then pick and choose your cuts in the meat department. You can get some very lean cuts without any perimeter fat and you'll end up getting more for your money.
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