ZW: zweiback
YT: byte
ZX = zzxjoanw
a word that has plagued lexicographers for decades. Ostensibly a type of Maori drum (or fife), it’s been labeled a hoax since Dmitri A. Borgmann’s Language on Vacation - 1965. It still shows up as a lexical entry from time to time.
ZY = fuzzy.
The last one is easy. I’ll hand it off.
ZZ: Zyzzyva. Also, ZY.
And that’s the end of the first round!
There’s not much left to do here, except to thank you all for your participation… But this word deserves a discussion by itself, as a sort if a post-mortem:
Now let’s do it backwards
ZA - zany
Backwards like ZA-ZZ, YA-YB?
Why not backwards like ZZ-ZA, YZ-YA, etc.? Players will get different words than they got in the first round.
Or first and last letters of the word, as in,
AA: armada
AB: Ahab
AC: almanac
Although it may be impossible… 
AD: ambled
AE: age
AF: aardwolf
AG: among
AH: awash
AI = alumni
AJ: swaraj
AK- Ask
AL = autumnal
AM: aluminum