Single letter call.

[W0AAA]

Z would be my choice.  Longer, but makes the callsign more distinctive.  K's at the end of a call can be confusing, even in a single letter callsign.

[XV4TUJ]

Hi,

Z, Q and W are good for both CW and Phone but usually in CW I prefer calls ending with a dash.
X (or Y) great for CW but not for SSB.
M is good for SSB but no so for CW (1M is too much dashes).

73,
Yan.
---
Yannick DEVOS - XV4Y
http://xv4y.radioclub.asia/
http://varc.radioclub.asia/

[SA3BYC]

I had this problem a year ago ... choose the X, SE3X to be my contest and DX call. Tell you it's worth every penny it cost when you can go with SE3X instead of SA3BYC ...

Works nice both on CW and SSB, on SSB though I do get some comments like "nice" call ... I do get that a lot from south EU stations and wonder about their view on my country Sweden and it's blondes    Do find the comments quite amusing and just recently made a new Qsl card design.   Click on the picture upper right corner to view:  http://www.qrz.com/db/SE3X  Haven't decided yet if I dare to print and send them out  

[AF5CC]

Quite a few DXpeditions have used C as a single letter suffix.  I can now see why.  Good on phone, and on CW.  Nice rhythm on CW, and for phone, the LEEEE part of Charlie gets high pitched and thru the pile up.

73 John AF5 Charlie Charlie

[K0HB]

M

[KW4M]

Definitely M.