Treats are not a reliable means of training a horse nor are they something that should be expected, in terms of how a horse behaves.
While I agree that there is nothing wrong with occasionally treating a horse (lord knows mine get treats far more often than they probably should LOL), I also agree that it's not a reliable training method, nor will it get the horse to "love you". If you want a horse's "love", you must first earn their respect by being an effective herd leader.
Basically, the only time any of my horses get hand-fed treats is when they are caught out of the pasture and when they are turned back out. I've found that treating them encourages them to be easier to catch in a large and non-enclosed space.
Other than that, if I feel the need to treat them (more for my own
desire than theirs LOL), I will put treats into a trough/bucket/pan and put it on the ground and make them wait until I walk away before eating. Only one of my horses is nippy and that's because my non-horsey Step-mom is completely enamored with him and I simply can't stop her from hand feeding him almost daily *eyeroll*.
As for what
to feed, I generally feed range cubes designed for cattle. Basically, what they are is a high protein forage substitute. They have high nutrient content and no sugars at all. The ones that I feed my horses are 20% protein.
While some cattle feeds are safe for horses, others contain ingredients that can be very dangerous for horses. So, for that reason, I suggest you find something that is made for horses. Alfalfa pellets make a nice healthy treat, though they can be a bit messy.
Whatever you get, avoid anything with molasses or sugars of any kind. There are much healthier options out there. Carrots are nice, but those are awfully expensive and have a very limited shelf-life.