Fenugreek, if you want fake maple flavor like Mrs. Butterworth.
Maple extract, if you want somewhat more realistic flavor, unless it's made from fenugreek, in which case nevermind, but you'll never know by reading the bottle or contacting the manufacturer, because it won't be listed in the ingredients, it will just say "natural flavors" which is oh so helpful.
You'll know fenugreek if you smell it. It is a sickeningly sweet smell and there's nothing else like it on earth (well except for Mrs. Butterworth's and her kin which are definitely made with it).
If you use fenugreek, please use much much MUCH LESS than you think you should use. Take your best guess as to how much you think you might need. Then divide that by about 10. That's how much you need.