I've recently started using ASI small icons. I've become a fan of it even though at first I thought it was terrible. While it's obviously not perfect, I think it does go in the right direction. I'd also like to say there are multiple solutions and that I'm open to a lot of ideas on improving the icons from the default set.
For the most part, I think the default unit icons are fine. Though I also like ASI's unit icons.
When it comes to structures though I prefer ASI. I just like how it handles highlighting important structures to know about including SML, SMD, TML, PD, etc.
I'm a fan of the colored highlighting ASI makes use of.
You can easily tell the tier of PD in this image. I do think the use of both the highlighting and the white bars underneath are too much though. There's no need for using two methods to show building tier at the same time. It should be one or the other. I also realize the colored highlighting can result in the icons taking up too much space and obscuring other information.
I also like the unit icons it has for air units, as well as structures like the TML.
It's super easy to tell that this is a TML due to it's red highlighting as well as the simple icon of a missile.
I know there's a lot of people against ASI, but I think it does have aspects that can be used in creating a new generation of icon sets.