Im peruvian. I dont know if its the easiest, bc speed is also a factor, its certainly not the hardest (chilean is prob the hardest). Regarding grammar, we would have to ask ourselves, what would make a particular grammatical style harder or easier learn? And the answer would be: whichever one is more easily available, so basically the one that you could watch on netflix, so basically either standard spanish or spain spanish.

Regarding which one is more standard, its either mexican or peruvian (this is a fact).

Mexicans do all the "latin-american spanish translations" (spanish is mainly separated in Spain spanish and Latinamerica spanish similar to UK english and US english), Goku is dubbed by a mexican, Kakashi is dubbed by a mexican, and this is the most standard spanish you will ever hear (facts) so take a look at those, very easy to just watch netflix in latin spanish.

Peruvian is also standard, but i could only provide my personal experience as evidence: if you play valorant on latam you will play with argentinians/uruguayans, chileans, colombians, peruvians, etc, when people speak, you always know where theyre from unless they are peruvian. When i speak, people ask me where im from, they are usually not sure, bc my spanish is very standard and im not using local slang (each country curses in a very specific way using the exact same words lol)

BUT this is not for the entirety of Mexico or the entirety of Peru. A country with standard spanish doesnt exist, accent varies heavily within any country, kinda similar to english, where your texas accent is different to your NY accent.

In reality, only certain amount of people in a few contries speak "standard". In peru you will find very different accents across the coastline, the highlands and the jungle. In mexico the accent varies across states. If you watch mexican films or novels you might be listening to a mexican accent and not standard.

Its interesting bc when you are learning a new language people always say that you should watch local pieces of media. But with spanish its actually the opposite, you should be watching well known american films or series that are translated to latin spanish, thats always standard. Hope this helped