Raichu. It's just plot that makes its power fluctuate widely. Tieing with Latios, and taking out Regice ( the latter being cheap but w/e) then losing to Elekid or something. Pikachu is pretty damn strong, but when the writers want it to fail it does so wonderfully...which is usually at the start of a new region.
Latios and Regice... basically, what you're saying is we should throw away Pikachu's power level in 650+ episodes just because in 2 (extremely badly written) episodes, it defeated/tied with a legendary pokemon (both times in totally unbelievable manner).
What makes the episode where Pikachu tied with Latios more representative of Pikachu's strenght than say Playing The Leveling Field where Pikachu lost to a gym leader's pokemon it also had a type advantage again. Oh yeah, it happened again 9 episodes later... Let's not even talk about his loses to Elekid, Onix, Magmortar, Luxray and all the matches where it couldn't do a whole lot but faint.
Also, it's really hypocritical to say on one hand that it's perfectly believable to have Pikachu defeat/tie with a Legendary pokemon despite being an unevolved pokemon and yet it's not ok for Hiyappu to do the same. Want to know what the difference between Pikachu tieing with Latios and Hiyappu defeating Pikachu is? In Hiyappu's case, its trainer had a strategy and didn't owe its win to crap writing.
It's not a question of not liking pikachu, it's a question of looking at this show in its entirety and realizing that Pikachu's normal level that's constant throughout the series is that it's strong, as strong or barely stronger than a Raichu, but not an invicible god that can kill things just by looking at them and is totally immune from losing.
Pikachu are frail. One wrong move on Ash's part with a clever trainer as an opponent and the tables can be turned against the beloved thunder mouse easily.
Besides, if Ash didn't get beaten at the start of every region to show he still has growing and learning to do then there wouldn't be much of a challenge in him just breezing through the gyms with six of his most powerful Pokémon.
Aside from the mascot thing, the reason Pikachu hasn't evolved is so they have an excuse for him to get beaten every now and then; even after his extensive experience.
Latios' back doesn't count as a strategy? (as if its any different than firing a water gun at someone's feet).
And I still don't know why you think it was crap writing. I can understand Regice even if I disagree with certain people's sentiments, but against Latios I'm not seeing how it was cheap. After all as I said, the only move that likely did direct damage to Pikachu was the final luster purge and even then that was probably only due to the explosion.
But really that argument aside, Pikachu legitimately lost to Hiyappu and it's previous wins/losses are rather moot. Corn just knows how to handle electric types, he wouldn't be a gym leader if he didn't have some strategy against them, he's not supposed to make it easy for the challenger.