I have seen several main menus in unreal engine but none of them look very good any tips?
Create a mock up or hire a concept artist to create one for you, then recreate the mock up using UMG.
The real question is, What k ind of main menu do you want?
Do you want a Static picture background with buttons on it?
Do you want a 3D background scene with buttons?
Do you want a 3D menu that when you click a button the camera moves between menu options in different locations?
Many many ways to do a menu, and from what you say, the ones you saw don’t look good lol.
So what’s your idea of good 
Let’s start there