"Please Wait"
If I were to speak to a friend of mine, I'd use chotto matte.

To say "Please wait" informally would be: ちょっと待ってください
How would I say it formally/respectfully?

"Omachi Kudasai"
or
''Shosho omachi kudasai''
or are both of those really wrong?

Context: Me speaking to a stranger on the street or in work, or even bus station.

Question

"Please Wait"
If I were to speak to a friend of mine, I'd use chotto matte.

To say "Please wait" informally would be: ちょっと待ってください
How would I say it formally/respectfully?

"Omachi Kudasai"
or
''Shosho omachi kudasai''
or are both of those really wrong?

Context: Me speaking to a stranger on the street or in work, or even bus station.

Answer

I work for a hotel as a receptionist. 
The word I use towards guests during work is 'Shoushou Omachi Itadakemasuka(Itadakemasudeshouka)?, which is very polite.  But it is strange to say when you speak to strangers on the street, at bus stations, etc.      Just ちょっと待ってください, Omachi Kudasai, and Shosho omachi kudasai are fine.