Hmm, I don't know if you have the Manna Pro treats where you are, but I haven't found a horse yet that didn't like 'em. I call 'em horsey crack!
My old gelding's favorite treats were carrots. When I got him as a 4 y/o he didn't even know what a carrot was, and I had a lot of fun trying out different things to see what he thought was yummy.
His preferences, from lowest to highest, were Manna Pro horse treats, apples, and carrots. He wouldn't eat anything else as a treat, and thought peppermints were my way of trying to poison him!
Until the day he died you could stuff him full of apples and horse treats, and he'd still look at you as if to say, "Okay, those were nice and all, but where are my CARROTS?!"