Synonyms for First class:

adj.

blue-ribbon (adjective)
banner, quality.
capital (adjective)
great, superb, best.
dandy (adjective)
swell.
excellent (adjective)
good.
first (adjective)
premier.
first-class (adjective)
first-rate, topflight, prime, superior, excellent.
first-class/first-rate (adjective)
choice, supreme, dandy, fine, tiptop, capital, top.
good (adjective)
bully.
prize (adjective)
topnotch.
superior (adjective)
choice, supreme.
topflight (adjective)
blue-ribbon, brag.

n.

board, booking, afloat, book on, aboard, ARR, bus, Booking office, bookable, aisle seat. a-one, capital, tiptop, quality, airmail, bully, brag, dandy, ace, express, banner, good, superb, blue-ribbon, tophole, swell, certified mail, boss, cash on delivery, first-rate, topnotch, terrific, prime, topflight, top, champion, COD, fine, great. choice, there is no substitute for something, ideal, best, incomparable, better, second to none, select, premier. passenger, jump a train, rail, class, shunt, coach, straphanging, railpass, second class. first-class (noun)
fantabulous, superior, first-class mail, priority mail, 1st class, excellent, 1st-class mail.
first class (noun)
fantabulous, superior, first-class mail, priority mail, 1st class, excellent, 1st-class mail.
special delivery (noun)
express, priority mail.

Other synonyms:

blue-ribbon, a-one, topnotch, tophole, certified mail, topflight, straphanging, airmail, COD, railpass, cash on delivery. first-rate, second class, incomparable, prime, express. dandy, tiptop, fine, select, champion. rail, shunt, great. boss, terrific, best, coach. quality. class. Other relevant words:
prime.