When I think about it at all, I think of the DJIA. It's quick, easy, and good enough. I have a palpable idea of what a given level of that index represents, solely because I have been aware of it for a long time (over 40 years).
I understand that the S&P or the Russell 2k include far more data points, and so are more representative of "The Market". But since I'm not a day trader (I'm more of a year trader), the usefulness of any of these indices is limited to indicating a big move which might prompt me to consider a rebalance. The Dow does that just fine.