Internally, the "accepted" strong syllable is the last (laTAM).. both are OK though, given the fact that it has a lot to do with how each language treats the word

Spanish and Portuguese consider LATAM to have a stronger second syllable, while Anglosaxon languages (English being the most relevant here) will have a stronger first syllable.

There's a TV commercial that was made on all three: Spanish, Portuguese and English. In all three (sadly I don't have links for the spanish and portuguese) it's pronounced laTAM
Check out the video here.. it's in the last few seconds: http://modocharlie.com/2016/05/el-nu...mbio-de-marca/

EDIT: this is NOT the same as posted a bit earlier in this thread, and sounds laTAM as well