Green mambas
Out of the three kinds of Green Mambas (Western, Eastern, and Jameson) that exist, all are highly venomous, aggressive, and unpredictable. Great. Furthermore all three have the characteristics of being highly arboreal, alert, quick, and agile. Additionally, their venom is much more rapid-acting AND the dendrotoxins are was more devastating to the central nervous system. No thanks.
South American Bushmaster
The South American Bushmaster is the longest venomous snake that you can find in the Western Hemisphere, like South America and the forests east of the Andes. Large, quick, and turns aggressive when cornered is what this particular snake boasts of.