Heroes 3.

The Platinum Edition (complete collection) includes all the standard campaigns and single-player scenarios. It also has a random map generator and the Conflux town. Overall, it's the best option for beginners. You can also customize things in The Shadow of Death, but it requires some effort, and I don't see the point in doing that. The only downside is the poor translation, but I personally play the English version.

By the way, the hardcore maps and campaigns in the standard collection are nothing special. You should get the author-made maps; they are much more challenging and interesting.