How should I store a static key pairs list?
Regarding Using variable_get and variable_set


I would suggest storing them in variables tables using variable_set in serailized format.. Store your 200 pairs data in an array using variable_get function...

Variables are cached in cache_bootstrap table so that performance will be improved.


Regarding Custom Table


You will lose the cache ability and managing the rows like edit/insert will be a pain since you need to build logic to retrieve and update results.


Regarding Cache Table


It will be a temporary storage. If some one clears all cache you will lose the data.


Regarding Constant in Code


Yes.. You can use constants too..  I am not sure performance impact vs using variables.


Regarding Hardcoded in code


I believe it's same as using constants. But, If you are trying to hardcode everywhere then you will end up with redundant code which will result in update - data consistency problem.

If you think in Drupal way i will say variables is the way to go using variable_get and variable_set..