I never liked 7 of 9 as a character that much. Yet another, and by this time tired concept of someone Who Discovers What It Means To Be An Individual Or Human - after several other series had done this already and even within the same series we already had the EMH on a similar arc of discovery. And then I'm not even talking about the way they costumed her yet.
None of this is the fault of the actress though, who did a terrific job with the character concept she was given and it's only thanks to her we got a character that was still worth to have in the stories.
There were other, perhaps far blander characters on VOY, such as Kim or later Chakotay, who didn't nearly get as much story material. But I had no problems with the concept of their characters, even if they were underutilized.
As for her best episode I might perhaps say Dark Frontier, when she comes face to face with her heritage and what it actually meant for her to be assimilated, and how everything was submerged under 'being Borg', so to speak. I'm just speaking about what it all would have meant for the character 7 of 9, not how plausible the backstory of the Hansens going after the Borg all alone, undetected for months using all kinds of fantastic cloacking and mimicking technology is in the first place, or the inconsistencies it causes.
I also like episodes such as infinite regress, but for a different reason - as she got to play a few very different personalities for a change and she does so convincingly.