SC2000 was a major breakthrough over the old SimCity, and it's importance and influence cannot be underestimated. But if you never played SimCity before - I would recommend starting with SC3000 - essentially the same, but much better usability.
This was an attempt to cross-breed old and awesome Transport Tycoon with slightly newer and awesome Rollercoaster Tycoon engine, since both game engines grow essentially from the same stem. However, the prime goal seemed to be making quick money instead of making a good game, and it shows. While it has some interesting innovations, it's just not worth it. Google for OpenTTD instead, download for free and enjoy!
If you have ever played the epochal Master of Magic, I bet you thought of something like "Ahh, the NWC guys are sooo good, I wish they took Master of Magic and brought it to life". Well, here you go! :) This is as close to MoM as you can get in HoMM: global spells actually can do something useful, heroes fight, the combat map view is essentially the same (albeit, turned right instead of left), wandering monsters! Unfortunately, by the time HoMM 4 was released most of it's core audience were HoMM3 adepts, so they just did not understand the spirit of those changes. Hence the image of "Heroes 3 with a twist". Plus the game was quite raw (use Equilibrium mod, use it!) and the graphics (after shiny and polished Heroes 3) were plain ugly (I especially love the bridge models). Still, to me HoMM IV was the pinnacle of the series, while HoMM 5 is kind of "Heroes 3 3D More of the Same for the masses". So if you want to play something that is like both Heroes and MoM combined - this is your best choice. The other would be Elemental (after it gets more patches).
It was a super-awesome game at its day, but I see no reason to play it after HoMM3 came out. Just for the sake of nostalgia and maybe the soundtrack. But then, HoMM3 soundtrack is by no means worse :)
