For the most part, all of the suggestions above seem pretty good to me.
IMO Brazil and Mexico are the usual candidates, for a few good reasons: territory, population. And right now (save for Mexico's drug war) they are both rising stars (Brazil more so than Mexico).
Had Brazil kept the monarchy, and its civil war never happened, it would likely have one of the world's largets navies (top 5) - making it an Atlantic power to deal with. Couple this with good economic decisions and relations with other powers and neighbors and you could also have a Brazil in the top 5 economies In OTL, Brazil is the 6th place in GDP and about 8th when adjusted to PPP. However, if you count it per capita it falls behind quite a bit. A "semi-wanked" Brazil could have:
A nominal GDP greater than 3 trillion (placing it between OTLs France and Germany) an that stays around there when adjusted to PPP.
When placed per capital it should have somewhere between 25,000 and 28,000 (a little under the EU average roughly equivalent to that of the Czech Rep).
This figures in a country in a country the size of Brazil, in terms resources and population. That in addition has a world class navy. Would make it a big power to be reckoned with. Raise, Angola and Mozambique (de-facto Brazilian colonies for a good chunk of their history) and you can even raise Brazil a bit more if it drains those two the right way.
In terms of Mexico; despite the fact that it is everyone's favorite punching bag in this forum (although there are a good Mexi-wank TLs out there two). Mexico isn't doing that bad in OTL, even when it hasn't lived up to its potential. It is still one of the largest countries in terms of population, 14th in terms of GDP and while it drops quite a bit when adjusted per capita it doesn't drop as much as Brazil's. Mexico also has a huge advantage over Brazil, which is the diversity of its economy. Despite being a relatively resource rich country, Mexico's economy is mostly centered around manufacturing not extraction (though it is still a huge part of it). With the right decisions, Mexico would have a larger territory. Say it still lost the Mex-American war, but survived the consequences much better off. Avoid the 2nd Empire, and keep Mexico semi democratic from the 1880s onwards, while avoiding the quasi-communist shenanigans of the PRI in the 60s and 70s.
Say it keeps everything south of the 35th parallel, and the Rio Bravo (let Texas keep the Pecos and nueces strip). This should allow it to grow, about 15% more in population, and much less centralized. You are also giving it another good port to the Pacific (San Diego later Los Angeles/Long Beach), some good farm land and oil in SoCal, more mining in New Mexico. Besides resource extraction and hard manufacturing, Mexico should also be capable of diversifying into a services economy as well without much trouble.
With the right economic decisions Mexico could easily bump up from being 14th on the list to about 8th (roughly the equivalent of Italy or Russia in nominal GDP), and maintain it as an equivalent to that of Italy or Israel when adjusted per capita.
It might not have the military equivalent of the "wanked" Brazil, but it would still be an economic power to be reckoned with, especially because of location. Unlike Brazil Mexico has two coasts.
OK, I wasn't planing for this response to be this detailed. The rest will be quicker. The rest of LA also has quite good potential.
Argentina was doing pretty good at the start of the XX century. Avoid some bad decisions and diversify its economy (it was terribly so, hence why it fell) and it could be pretty well off in the present. And Chile is doing relatively well for a country its size in OTL, and it certainly had potential to be even better off.
However I think here you get into a matter of balancing, I don't think you can have a strong Chile, Argentina, and Brazil at the same time (let alone adding Paraguay to the mix). A strong Argentina is an automatic rival to Brazil and Chile - you can still keep Argentina from falling, but i think in a full LA wank Brazil and Chile will keep its growth in check.
Similarly a strong Mexico might try to keep the UFCA from forming again in the late IXX century (it did so in OTL).
Colombia could be a good addition to the mix, especially in the form of Gran Colombia. Its population would be near 100 million, but probably not quite there. It would certainly benefit from Venezuelas oil reserves, and if keeps Panama, the canal. However, unlike Brazil and Mexico, I think you run into the issue of Colombia staying as an extraction economy (both Colombia and Venezuela are in OTL) much more than OTL. It could be a natural resource powerhouse, like Russia and Canada are in OTL and still be "wanked" but it is harder to make good economic decisions then.
As for Peru/Bolivia, as much as I like the idea of a semi-native Quechua state that dominates South America. You do run into quite a bit of geographic and demographic problems that are difficult to get rid off. At its best it would still be the smallest of these countries with the exception of Chile, (roughly 50 million pop), and like in OTL with distinct geographic identities beween lower and upper regions. And once again you have the matter of balance, a strong Colombia and Chile will try to keep it in check. Chile did so in OTL. And it would even be competing against Brazil in the Amazonian regions, and Paraguay or Argentina in the Chaco depending how they are doing.
However bare in mind that rich neighbors and friendly relations between them, usually also help bring up the neighbors. If Colombia, Brazil, and Chile are all doing much better than OTL. Their trade should result in Peru doing better as well. If Mexico and Colombia are doing good Central America (divided or not) will also benefit in the long term.
So yeah in summary Latin America has quite a good potential, some of the countries were doing pretty good before screwing up (Argentina, Brazil, even Paraguay), some have been doing relatively well for what is available to them (Chile, Costa Rica, Uruguay), some haven't quite met their obvious potential (Mexico, Colombia), and some are slowly but surely picking themselves up and catching up (Brazil, Mexico). No reason why the whole region can't be wanked.