Normally you might be right, but Keeper II is Helloween's most famous and classic album. If you really don't want to, start with Keeper I or Walls of Jericho (their debut).
Better yet, if you want to start with only one track, check out Keeper of the Seven Keys off the Keeper II album. It is perhaps the greatest showcase of everything power metal ever was. In fact, I personally think it is the only power metal track you ever need to know... either that or Rebellion in Dreamland by Gamma Ray (same singer, incidentally).
But don't pay too much attention on my personal opinion. I don't like Helloween or power metal very much. Among other things, I don't like the singing. I understand everybody loves Michael Kiske, but I personally have never understood why. Not only is his shrieking so high that it hurts my ears (and I have never found any appeal in that), but I really can't tolerate his heavy German accent either (I know that sounds strange with me being German myself, but it's true). But if you like high singing and don't have a problem with the accent (or perhaps even find it exotic), then go check them out. As for Kai Hansen, I can bear him a little better, but I have never been able to take his voice seriously either. To me, he sounds like a character out of a children's cartoon.