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
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