What is the fastest way to get Zenith in Zenith Inc?
The documented answer: keep the generation-and-upgrades cycle running without gaps. The official description names Zenith generation as the first action and Upgrades as the multiplier on it, so the fastest verified route is spending Zenith on visible power gains the moment they pay for themselves, then climbing to surface the next tier. Exact rates are not published anywhere, so no source can honestly quote a per-second figure.