Hi,
In preparation for the UMAT, I did medentry for three months. It had heaps of drills and practice questions, but also practice exams. The guides were somewhat useful, and whilst I didn't read all of them, I definitely did learn from a few of them. I had a few friends who did iCanMed, and heard good things about the way they taught students how to tackle questions. But, they don't have practice exams, which are so important, because it can be a real struggle to finish the exam in 3 hours. One of my friends used iCanMed, but ended up also buying medentry because he wanted practice exams. I've never heard of hellouni, so I can't comment on that.
I would recommend medentry because it is the most trusted and experienced program. It is expensive, but if you are really set on medicine and can afford it, you should really consider it. You want to give the UMAT your best shot.