I chose suspension first. Mostly because the suspension in my car was the area most in need of attention. It had two blown struts, bushings were in bad shape, and undersized tires make it look like it's on stilts. The engine leaks some fluids, but is basically competent.
Think about it this way: a decently handling but underpowered car will be fun for a little while, but a poorly handling overpowered car will be scary every time you drive it.