How to choose the best:
Celery

Touch

When picking out celery, look for a vegetable with a head that feels firm and tight, with nice strong stalks and robust leaves atop.

Keep an eye out for limping or rubbery stalks, as well as wilting leaves — signs that the celery might not be the freshest.

See

Look for a a vibrant color along the stalks. Towards the top, nearer the leaves, the color will be paler, but it still should be bright.

Keep an eye out for limping stalks or wilting leaves — bad signs.

Smell

Look for a faint smell, especially from among the core and the leaves.

Some more details:

When it's in season

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

FYI: for this Vegetable, its best to buy organic.