S&P 500. It's roughly 4/5 of the total US market by capitalization, and it's simply capital weighted.
The Dow Industrials is the sum of the prices of one share of each of the companies currently in the index, times a 'multiplier' that adjusts for changes in the exact companies that make up the index. It is loosely coupled at best to the broad stock market.
The NASDAQ Index is a motley selection of non-financial companies heavily weighted towards 'tech' whatever that is, with a bunch of arbitrary multipliers cut in and out as companies change size, so as to meet the whims of fund managers. Yeah. That. Anyone remember NASDAQ 5000?