After watching some of the cartoons on the Speedy Gonzales DVD: Does anybody agree that Sylvester was a poor choice as an adversary for Speedy?
It seems like when Friz Freleng picked up the Speedy character and turned him into a viable series character, it almost seems like he picked Sylvester out of habit, because that's the cat character he always used. (Jones and McKimson would alternate between using Sylvester and other, original cats, but Freleng brought in Sylvester any time he needed a cat, in almost any context.) But there were still some cartoons after that where Speedy went up against new adversaries, and these are almost always better: the big Mexican cat in "Tabasco Road" (McKimson), the vulture in "Tortilla Flaps" (McKimson again) and Jose and Manuel in "Mexicali Schmoes" (Freleng recycling his "Two Crows From Tacos" characters, whom he would eventually turn into the Tijuana toads). These characters actually add to the humor with their fractured Pepe Le Pew-ish English and their exchanges with Speedy; by comparison, Sylvester seems out of place in the Mexican setting and not very funny.
And yet after "Mexicali Schmoes" Sylvester was more or less Speedy's permanent adversary until they replaced him with the even-more-inappropriate Daffy Duck. (As usual, McKimson appears to have started out doing something different and finally going along with what Freleng was doing; McKimson didn't make a Speedy-vs-Sylvester cartoon for several years.) Speedy was never a great character, but pairing him up with a villain with whom he has nothing in common and no real chemistry -- whether it's Sylvester or Daffy -- makes his cartoons much duller than they might have been.
On the other hand, the choice of the "gringo pussycat" as a villain may have contributed to the series' popularity in Mexico, since instead of a Mexican equivalent of the Pepe Le Pew cartoons, the series became the story of a heroic Mexican constantly defeating a villainous American. It probably helped deflect charges of racism and helps explain why the Speedy cartoons haven't been as controversial as other racial-stereotype cartoons (except for that period when Cartoon Network decided they needed to ban him).