What is the world's largest herb?

The banana plant

http://homepage.eircom.net/~brianfleury/arcfctnov2000.htm

Also considered the "tallest herb"

Banana

Well you learn something everyday - I agree with Quizzer - The great banana, considered 'fruit of the wise man' in India, is in fact not only delicious but the tallest herb in the world, with trees sometimes growing to a towering 25 feet tall.
https://en.wikipedia.org/wiki/Banana#Description

You are so right SOHC, who would have thunk it! Here is a little more information
for anyone that might be interested.

The banana is a herbaceous type of plant and bears fruit at non specific times
of the year. Most of these huge herbaceous plants grow on average of 15 to 30
feet tall. See:
https://web.archive.org/web/20090415160027/http://www.hort.purdue.edu/newcrop/morton/banana.html

The banana

When I went to the page to check the link and found the answer was a banana, I did some further checking because it was my understanding that the banana was a berry. Here's some of what I found.

Is a banana a fruit or a herb?

Both. A banana (the yellow thing you peel and eat) is undoubtedly a fruit (containing the seeds of the plant: see answer regarding tomatoes), though since commercially grown banana plants are sterile, the seeds are reduced to little specks. However, the banana plant, though it is called a 'banana-tree' in popular usage, is technically regarded as a herbaceous plant (or 'herb'), not a tree, because the stem does not contain true woody tissue.
https://en.oxforddictionaries.com/explore/is-a-banana-a-fruit-or-a-herb

What is a banana? Is it a fruit, an herb or a berry?

Well, bananas are all those things. The banana is the most confusing item I have ever come across in produce. What's so confusing you ask? The confusion is in respect of the definition of 'Banana'. The Banana tree itself is an herb. The best definition of herb as far as I know is this: Herb is a flowering plant with a fleshy rather than woody stem, which usually dies back at the end of each growing season. The Banana tree trunk is a false stem formed by tightly wrapped leaf sheaths. The Banana itself is from the berry family. A berry is a simple fruit having a skin surrounding one or more seeds in a fleshy pulp. If you cut a banana in lengthwise, in the center, you will notice very tiny black 'seeds'. So, the Banana is a fruit, herb and a berry. I wonder if that's where the saying 'going bananas' comes from.

http://comevisit.com/chuckali/banana.htm

Technically speaking plantain and banana plants are the world's largest herbs. The plant is of the Muscaceae, the largest of tree-like herbs grown for fruit, as well as large leaves and fibre stalks.

https://en.wikipedia.org/wiki/Banana