If you're talking about within the anime, I loved chubby Pikachu more because well...he's chubby, and I like those kinds of things. He had less moves in the anime, so Ash really had to strategize in regards to how he used the terrain and what not.
Athletic Pika is in no way a bad version, I just like chubby Pika more. Oh, if I recall, chubby had more quality screentime, so the moments he did appear were stronger in emotion. That's how I feel.
I honestly perfer the chubby little mouse creature that we saw in the games and artwork. It always seemed to me that it just clicked and fit him very well. When I saw the little, skinny, atheletic Pikachu I was surprised because it just didn't fit with him. It kind of reminded me of Sonic the Hedgehogs appearence change. I think that a lot of people can agree with me that the little chubby Pikachu is the better design.
I love both versions of Pikachu, but I prefer chubby Pikachu, mainly because I grew up with the guy. When Pikachu was chubby, he seemed to walk like a duck. There was also more emotional memories with than with athletic Pikachu. One memory that I'll never forget when Pikachu started to obey Ash after Ash protected him from a group of Spearow in the first episode of Pokemon! I felt so touched to see that scene and I never forgot it. Chubby Pikachu also acted more silly than athletic Pikachu. When they were in Dark City, I always loved it when Pikachu was licking the ketchup bottle from the top!
Pikachu has certainly changed over the years. Really, just look at the sprites:
...Yet I prefer chubby Pikachu. The original Pikachu wasn't as serious and was more curious, and he just looks more classic. (Like how I prefer the classic Mickey mouse designs to the new ones. More classic.)
I like chubby Pikachu more for three reasons-1. it really gives Pika more funny moments. 2. he's probably more huggable. and 3. CUTE!!!.
I think the Anime should bring the Chubby, funny,and much cuter Pikachu style back.