It's the best of its
Notice that there are two ways of spelling its used here - follow the link to find out why.
more information: its & it's
frowned upon
Something that is frowned upon or frowned on is disapproved of.
Example: Spitting in public is frowned upon.
Ran is the past tense of the irregular verb run. Follow the link to find out more and to listen to examples.
more information: run
build up
gradually increase; accumulate
Example: Silt builds up in the dam.
For more meanings of the phrasal verb build up, follow the link.
more information: build up