A final boss always has a big effect on your feelings about a game once you've finished it.
Despite the fact it usually takes a relatively short amount of time to complete, this ultimate encounter is one of the most precious parts of the game; an intense amalgamation of everything you've experienced up to that point, but with more explosions, more emotions, more challenge and more frustration.
Unless you're an obsessive completionist, then the fact that you even made it to that final showdown means you're invested in seeing it through to the end. As such, finally landing that finishing blow should trigger the most positive of feelings, alongside a certain sadness that your adventure has come to an end.
But, if a final boss fails to live up to the challenge that you expect from it - if it doesn't do justice to all the time, skill and dedication required for you to reach that point - then it's always a case of any previous goodwill being replaced by a blank look and a bemused "Is that it?" response.
While this rarely ruins an otherwise great game, it can definitely taint it. There have been plenty of brilliant games over the years that failed to end on a high note due to bosses that just weren't worthy of their assumedly prestigious status, so let us know which you thought almost tanked their respective games in the comments below.