Style Guide for Z80 Assembly Code

I think it's mainly a matter of taste.
There is no universal conventions about coding for Z80.

This is my style :

The most common style I've seen is: labels aligned to left margin; 1 tab before instruction, 1 tab between instruction and parameters, no space after comma between parameters, variable number of tabs after last parameter and before the comment (if any) to align all comments together. Pretty much like Metalion's example. When a label is too long to fit in the first 7 spaces, place it in a line alone.

However, I like to use two tabs before the instruction, because it allows for longer labels before needing to place the label in a separate line.

Some Z80 coders follow style enforced by the native tools back from the day, for example in MRS the line was strictly split into columns, labels starting at beginning of line, instruction starting somewhat middle, and arguments further down and finally comment toward end of line... (ie. quite close to what pgimeno described, although "tabs" were not at 8, but at defined positions)

But I wouldn't dare to say "most common", especially now there are many different styles around, depending mostly on personal taste, although some assemblers have requirements as well.

Totally agree with the personal thing!

If there is a standard, the most important thing to me would be that it controls the syntax to be used to maximize portability. I've reused code written by other people, and there's usually one or two gotchas of code that compiles when moving it from one compiler to another, but that it is actually interpreted differently (I give examples in the video I mentioned above). So, I'd be all for some "general recommendations" on what and what not to do at the level of syntax (e.g. colon vs not colon in labels, indenting labels or not, etc.), to minimize these gotchas.

But I would not like to tell people where or where not to put tabs, etc.

Well, I'm talking about code I've seen back in the old days

Starting with the M80 Assembler manual, which may have been influential in propagating that style. The Complete Spectrum ROM Disassembly uses that style too. Also seen in code for 8086 by Borland, by Microsoft and others.

Edit: I haven't talked about letter case. Most of the code I've seen used upper case for everything except comments or text, but that's not a convention I was fond of. I used lower case myself, with labels in Pascal case (aka UpperCamelCase).