Round one for sure. If nothing else, there's pretty much always at least one game that's really going any given night. Just by sheer volume. But you also get that fresh out the gate excitement level, playoffs still feel shiny and new and most of the teams are coming in at full strength. Plus the potential for some surprises and back and forth matchups that you didn't expect, or blowouts that you didn't see coming. Lots of storylines to follow all over the place and often the best actual hockey before everything gets ground down and dragged into the muck.
It's particularly pronounced with this current playoff format. More and more, the best matchups under this playoff seeding tend to happen right off the line as the playoffs open. The later rounds just end up feeling like "the battle of whoever's left". Didn't used to be that way. At least it feels to me like we used to get a lot more terrific matchups deeper into the playoffs where you're just waiting for the titans to square up.