There are plenty of digital editors for pure ST, however we found two of them really useful.

ProTracker ST
(The archive includes also a version for Atari STe but that's another subject).

It works with FDD but also it works instantly with HDD and CF/SD cards which is cool.
It can play 4-channels digital music with two alternative qualities:
- 12 228 Hz (this quality is not great but if You have only 1Mb of RAM and wanna make any bigger song - You will have to use the quality to play the song at all).
- 16 384 Hz (that's really nice quality for making digital music on our ST, especially if You keep filled up all the tracks, because then it sounds much better - noises coming from sound-quality are LESS audible).

You have to remember that ProTracker ST doesn't read 'tremolo' command, so if You wanna use the effect, You simply have to set up twice faster tempo to achieve the tremolo if required.
The editor contains SAMPLER corner and You can prepare Your own samples using any sampler-interface for ST. Supposedly like this one:

The second prog is DigiComposer.

Also 4-channels of digital sound. Sadly works rather only with FDD. It gets crash while work with HDD somehow.
There's some confusing PREFERENCES corner but when set up properly - it sounds better than ProTracker ST.

In my opinion it is generally better than PT ST but seems like DigiComposer might have troubles with "jump to pattern" command.

Both editors use MOD format. As for me the sound quality is really OK for Yamaha and for the additional channel from software-mixing. 

Probably the best of them all, on many aspects, is the HexTracker. 

This is a real monster among Atari ST (and also STe/Falcon) trackers. Max. amount of channels: 16!!!!! (On YM chip!!!). Of course it works also on higher models of 16/32bit Atari, with better quality. About the problems - at the moment it has no fine-tunning of samples and no self-repeating keyboard/mouse (yes - painful). 

It is worth to make also digital music on Atari ST. :)