This is the 467th most frequent Estonian word.
suurim
Largest.
In this sentence, 'suurim' is used as a superlative adjective to describe the noun 'unistus' (dream), indicating that it is the greatest or most significant among others.
See on minu suurim unistus.
This is my greatest dream.
Here, 'suurim' characterizes the noun 'rõõm' (joy), expressing the idea of being the most intense or fulfilling joy.
Suurim rõõm on jagatud rõõm.
The greatest joy is shared joy.
In this question, 'suurim' functions as a descriptive word for the noun 'jõgi' (river) in its superlative form, used to ask about the river with the largest size or scope in Estonia.