For DS9, seasons 1-3 can be watched out of order, 4 and on can still mostly be understood and enjoyed out of order, but plot starts to build. Im sure somebody on reddit has compiled all the plot-essential episodes if you really want to make sure you hit them all.

The thing about DS9 is that the fillers and one-offs can be really good (or terrible!), and rarely need the plot episodes to make sense of them.