I love coinmarketcap most of all. It has all the main coins listed and you can see general charts as well as current prices on different exchanges. There are lots of ways to sort coins on this website, which is helpful to get the general idea about the market. For example, apart from the list based on market capitalization, it is really useful to take a look at 24h biggest losers and winners. Bitcoin's dominance in the top of the page is helpful to get the general idea about the dependence of alts. It's also amazing to explore historical snapshots since 2013 to see how the market was evolving and which investments kept turning out to be successful.