When I first read this prompt, I thought "dear lord, which one?" When I speak of exes I generally mean one of two people - my ex-husband or my most recent ex. Those conversations are either the mummified remains of an ancient thank you for the two best gifts I've received or too soon.
Only one other "ex" is someone I still refer to - often here, in glimpsing and side-long ways - but never by that moniker. His former status is subsumed in his current roles as my Jiminy Cricket, my confidante, my champion. He is the voice I call to mind when I forget who I am. He taught me what it looks like when love waits its turn; then when I was ready he taught me what it is to love freely, passionately, and compassionately. He taught me the joy of sex. He taught me the joy of "you never know." He taught me that you can be fully for someone without owning that someone. There is nothing to write to him, because there is nothing I need to say to him that I haven't already said. The space between us is vast, but clean, and crossable in a heartbeat. He is in his own category that cannot be contained in a prefix.
There are, of course, other kinds of exes. Ex-bosses, as Shiney pointed out, though that gets problematic in its own right. I've had many bosses who taught, lead, believed, coached, cajoled, trusted, and helped me to grow. They know who they are, they know what they did, and they are still in my life. The one who isn't, well, I don't think he could understand or care about anything I have to say, just as anything but an unqualified "sorry" from him is of no interest to me.
So, this prompt has done its job in that this post now exists. But a letter to my ex? Why? The ones who need it will never see it, and the ones who will see it know all there is to say.