The builder by default is assigned to hot key 1, which is great. I would suggest to additionally assign the elemental summoning center to hot key 2.
I guess most users mainly only interact with the builder and the elemental summoning center (of course people select towers and target manually sometimes, but usually they don't assign hot keys to them; well, at least that's how I'm playing it), so having the building available on hot key 2 would be a reasonable default.