It's not exactly rare for the final boss of a game to be a bit lacklustre. If you've done all the optional extras the game provided you with before getting there, chances are whatever ancient evil or world-ending force you have to fight against provides all the challenge of a slightly damp linguine strand.
And, even if it does, you're likely very, very prepared for it. All the hordes of valuable items and power-ups that you've been holding on to (and refusing to use even when you needed to) are for this one battle - and since it's the final battle, you suddenly have full license to chug every revive you've collected along your way.
With all this in mind, there are plenty of times where your biggest challenge was actually the strength of some of the regular enemies you fought along the way.
Despite the fact they were only supposed to be stepping stones on your route to this big bad, when you look back it actually seems like they were far more dangerous - be it because you were ill-prepared, or because they were downright powerhouses.
Sometimes - in life and in video games - your greatest challenge isn't always what you think it will be. Sometimes it's just a lizard.
10. Whitney - Pokémon Gold/Silver/Crystal
Fighting Whitney for the first time is a sort of cultural zeitgeist. If you did, and you went in totally blind knowing nothing about the fight, you were likely in for a pretty bad time. Whitney's Miltank is the most OP enemy Pokémon you'll see for some time, as not only does it have a health pool higher than anything you've seen before, but its ability to use the move Rollout makes it a prime candidate to wipe your team.
Now add to this the fact that this cuddly looking nightmare spawn can also use Milk Drink, which heals it. So the concept of just whittling it down is also kind of out of the window, as you'd have to do it so many times that this cow stopped ran out of milk. Which, come to think of it, was actually a kind of messed up concept in the first place.
Even if you have a set tactic against Whitney - as you can trade for a Machop that will usually decimate it - this doesn't counter her Clefairy, which can use Metronome, enabling it to randomly pick any other move in the game to use. And if that happens to be Hyper Beam? Your only reward is our condolences.