In terms of viewership, T20 is the most popular format these days. Cricket purists tend to favor test cricket though, mainly just because it's the original format and requires certain skills which aren't as important in the shorter formats. But you can understand how a format which can last up to 5 days is a little less accessible to casual fans.

Test cricket is a game of patience where batsmen are encouraged to be very conservative and ensure that they protect their wicket (don't get out) even if it means scoring very slowly. In the shorter forms, batsmen are encouraged to be much more aggressive and prioritize their run rate.