My choice is Rival Turf by Jaleco on the SNES. I used to read video game magazines for reviews back then and they totally slaughtered Rival Turf with scores like 4/10 for being crap and also being a Final Fight clone. But when I played it with my brother, it was so much fun we played for hours. It was my brother's friend's game, but after playing it, we ended up buying our own copy. I was really young, so looking back, maybe I didn't know the factors that made a game good or bad, but since then, I stopped paying so much attention to game review scores. Sure I still read the reviews but if a game looked fun, its review score was irrelevant.
What about you? What was the first game you played that made you not trust game reviewers? It could be a game that got poor reviews, and when you played it, you found it to be very good. Or it could be a game that got great reviews, and when you played it, you found it to be boring and not so good.