North Star with a 25 gallon tank. The only thing I didn't like about it was the mounting straps were almost impossible to tighten and keep tightened up. I put a couple of cheap ratchet straps on instead.
I've had a Northstar for 12 or 13 years. The tank will probably outlast me, but everything else is replaceable. I still have the original pump and hoses, but have replaced a lot of parts. I replaced the straps and modified the mounting system for the nozzles. I bolted them to the rear frame of the ATV and it works better than their system that mounted to the rear hitch.
I don't think you can go wrong with the Northstar, but don't be afraid to modify it to fit your ATV.