Which is the most proper way to sign a letter? John Smith Esquire, John Smith Attorney or John Smith Attorney at Law.
Besides, does each of those categories denote different levels of engagement in the legal profession?
Esquire is a form of address used in different ways in the U.K. and the U.S. In the US, it is only used as a professional designation for a lawyer. Emily Post wrote,
This applies to the US. In the UK, the term is a bit broader:
So, if you are in the US, you can sign a letter John Smith, Esquire or John Smith, Esq. This is the form that I have seen most often in the US.
The terms attorney or attorney at law are names of one's actual position. For example, the editor of a magazine might have a byline that says "John Smith, Editor-in-Chief" and similarly a lawyer might use John Smith, Attorney at Law.
Since all of the terms are different (Esquire is a form of address, attorney is a position) they don't designate different levels of engagement in the profession. An attorney at law is an attorney who can appear before the court.
Note that some lawyers do sign using "Esquire", so if the lawyers around you are doing so, feel free to do so as well.
To address a letter, use Esquire or Esq. in place of "Mr." or "Mrs." To sign a letter, use Attorney or Attorney at Law since they denote one's position rather than one's proper form of address. To sign with Esquire would likely be like signing a letter, "Mr. John Smith."
The use of "Esq." and "Esquire" in reference to attorneys or attorneys-at-law should be extinguished. In the UK, attorneys were referred to as barristers, and "important" men or men in a high position were referred to as "Esq." and that could could and probably did include barristers. In high school nearly 45 years ago, we were taught that "esquire" is archaic and comes from England and means "Mr." or "Mister." Defining "esquire" as "attorney" would imply that Esquire magazine is written only for attorneys, and it is not. It is written for misters. It is so disheartening that the use of "Esq." was resurrected from uncommon usage maybe about 20 years ago. I have worked for attorneys since high school to see the uncommon usage become popularized. In my early days of working for attorneys, female attorneys were few and far between--only two in my small town. After women's liberation, the number of female physicians, attorneys, etc., began to grow. It is so disappointing to see the improper understanding of the definition of "Esq." to be passed on to female attorneys. Only in the last five years or so have I started to see many attorneys (male and female) properly referred to as "J.D." (Doctor of Jurisprudence) since it is their degree and does mean attorney. If one insists on the use of "Esq.," then it should at least only be used to address someone else and never following your own name (if one is an attorney) and please never after a female's name. To bury "Esq." in antiquity where it belongs has become a lifelong quest for me primarily because it does not mean attorney. To all those who may read this, please help eradicate the improper and archaic usage of "Esq." The common misunderstanding of the definition of a word should not be an excuse for it to be accepted as correct usage.
The below answer contains incorrect information. Etymological/philological concerns notwithstanding, the use of "Esq." as a means of (written) address for attorneys in the States is absolutely correct, regardless of gender.
The designation "JD" refers to the degree, Juris Doctor (not Doctor of Jurisprudence, which is JSD). It means one has a U.S. law degree, period. It has nothing to do with whether he or she is an attorney, since attorneys are only those who are licensed to practice by the state bar or other administrative organ of the highest court of the U.S. state in question. Getting licensed usually requires passing a bar exam, moral character/fitness investigation, etc. Thus, there are many "JDs" who never become attorneys because they can't or don't wish to satisfy the requirements necessary to actually obtain a license to practice law. You certainly would not refer to these people as "Esq." (and they had better not instruct others to address them that way unless they want to be accused of the unauthorized practice of law!).
Please note that it neither would be appropriate to address licensed attorneys as "JDs" for a couple of reasons. One, because of the distinction between "JD" and "Esq.," others may infer that the lawyer isn't licensed (i.e. not really a lawyer). Two, believe it or not, there are still states which don't require one to have a JD in order to sit for and pass the bar exam (though the chances of anyone without a JD actually passing a bar exam is extremely remote nowadays). Thus, technically, not all Esquires are JDs.