I love Pikachu, I thought they were overrated for a long time because of course, being a mascot character they oversaturated Pokemon merch as a whole. But now I think Pikachu is really cute and lovable. I think a big part of it is Pikachu's VA, Otani Ikue, who made Pikachu's voice so memorable and iconic.
I think it should either have been Rhydon (which they were going to do in 1993) or Clefairy. I like Clefairy and it has generally broad appeal. On the other hand I'm kinda glad it was Pikachu so that neither of them got oversaturated.
Another Raichu lover here! Best Pokemon <3 But Pikachu is cute, too, and has a cool appeal for young boys which helped make it universally appealing to children, so I really wouldn't have had it any other way. Clefairy has always seemed like it was too pink and cutesy to achieve that. I think it's part of the reason Kirby didn't sell so well in the States but sold quite well in Japan.