Obviously this will vary from person to person - but there's something about the last third of Season Three to the end of Season Four that I just love. The end of Season Three in particular beginning with Before And After is a period of Voyager that I adore - Displaced, Worst Case Scenario, Scorpion. The series really seemed to hit it's stride in late Season Three in my opinion and this continued into Season Four. Boundaries seemed to be getting pushed with episodes like Year Of Hell, Scorpion, The Killing Game etc. The look of the show also seemed to improve significantly as CGI became more commonplace, allowing aliens like Species 8472 to be realised and also shots of Voyager herself to be showcased in a different way. I'd say this sweet-spot (again speaking personally) continued into Season Five, with Timeless, the sublime Counterpoint but with Dark Frontier I feel there was a sea change where things started to become a bit (and I appreciate how silly this sounds given the nature of science fiction) unrealistic. Getting the better of the Borg time after time, things becoming too centred around Janeway, Seven and The Doctor. For me late Season Three and Season Four is my favourite 'era'.