great vs peachy

great is a noun but peachy is not a noun.

Word	Noun	Adjective	Verb	Adverb
great	Yes	Yes	No	No
peachy	No	Yes	No	No

As adjectives, great and peachy are synonyms defined as:

Other synonyms of great include bang-up, bully, corking, cracking, dandy, groovy, keen, neat, nifty, not bad, old, slap-up, smashing, swell.

great (noun)	peachy (noun)
a person who has achieved distinction and honor in some field

great (adjective)	peachy (adjective)
excellent	of something resembling a peach in color
in an advanced stage of pregnancy	excellent
uppercase
remarkable or out of the ordinary in degree or magnitude or effect
of major significance or importance
relatively large in size or number or extent; larger than others of its kind