I'll argue that "Lifesigns" is essential because Denara Pel comes up again in "Resolutions" and refers to the Doctor by the name that she gave him in "Lifesigns". "The Thaw" is a great episode and features one of my favorite actors Michael McKean, however I wouldn't call it essential. Just highly recommended.
I would argue that "Resolutions" is essential if you had any hope of Janeway and Chakotay getting together!
And "Threshold" is essential for mocking purposes. Come on!