I played SM, or AOW 3, and i loved it.
I heard the basically each sequel is better than the last in everyway.
Any reason why a SM player should try part 1?

In my opinion it's the best in the series. I listed some of the reasons in the thread named "Buy the first one now or wait for the last...".

If you loved part 3 I think trying this is a good idea.
Part 2 is just an earlier version of number 3. Number 3 is a stand alone expansion.
Number 1 is a different game. Many things changed in 2\3. Some races were removed (Azracs, Lizardmen, Highmen) Many of the units were take out and new ones got put in their place.
In number 1 a hero could choose from a huge list of abilities\skills when they leveled up so you could customize a hero more.
In number 1 units could have physical immunity. The Incarnate was a much more fearsome unit since it had physical immunity.

Reasons?
- much, much better campaign - there are multiple paths and endings possible
- significantly better music: Carved in Stone, Love and Death, Battle Macabre and many others
- bigger hero customization
- more detailed lore and better feel
- more players can play on one map

The music is perfection, also the campaign is very very good.

You know you can resize and move every single UI window, right?

Thanks, didn't know that.

You can also change to the resolution of your choise, including widescreen resolutions.
I prefer a low resolution for my 14,5 inch monitor, but if you have a large monitor and want to get a really wide overview you can.

wow ok you sure can.

i like to keep res low on most old games.
dont want tiny little 2d sprites.

Or huge blurry textures. Yeah, I do that as well.