Most of the time, Tuvok was just kind of there. Sometimes he was featured as one of Janeway's closest friends and confidants, but sometimes he was just kind of there. As tactical officer, he sometimes got to play cop, but he never really got to be the House of Star Trek, even though he was ideally suited for that role, logically explaining things no one else would ever have even considered, with just a vague note of sarcasm in his voice. (Seriously, how awesome would that have been?)
As the second Vulcan to be a series regular in Star Trek, Tuvok had a lot to live up to, but not until T'Pol did the writers feel like really contrasting the Vulcan perspective. Tuvok was a nearly ordinary Vulcan, who after all still had his share of unfortunate repercussions from being lost on the other side of the galaxy.
And then there were the episodes where the writers just wanted to play cruel tricks on him. "Riddles" is a cruel trick. Of course, Tuvok counterpoint Neelix is there to help him navigate the experience, such as possible. Tuvok and Neelix were the unsung heroes of the series, the ones who most grew in relation to the other. This is the one time where Tuvok really seems to appreciate the relationship, and it's because he's lost pretty much everything that makes him who he is.
It's tragically awesome. You don't need to be a fan of Voyager to enjoy this one, and it could have been another species besides a Vulcan who suffered in it, but it's the definitive episode for Tuvok and Neelix (and there were some pretty good rivals, not the least being "Tuvix"), and will have you crying salty tears a season later when they say goodbye to each other.
franchise * series * essential * character
Memory Alpha summary.