Best software for printing letter characters?

I need an expert to recommend the best software to create STL files that contain text (words and letters).

I need to be able to mirror the letters in order to produce blocks similar to old-fashioned typesetting.

"Best" is a rather subjective attribute, especially when no reference is provided. Just about every 3D modeling program has the ability to create text for STL models. In my opinion, Tinkercad will be the easiest to use and the fastest, but also the most limiting when it comes to typeface selection. On the other end of the scale, you'll find programs capable of the task but poorly suited. Again, it's just one person's opinion, but ZBrush and Blender can do it but it won't be easy to learn. In the middle, I find OpenSCAD to be the best program for me, while you'll find many users like Fusion 360 and OnShape. The mirroring aspect is possible in all of these programs.

Excellent! This is exactly the information I need. By "best" I meant a program where creating text is just doable. I'm not deterred by the learning curve. I'll give each program a try. Thanks.

If you're doing rubber stamp type creation, Fusion 360 will provide a tapered extrusion if required. It is a free program if you are able to find the correct link for the hobbyist version. That version expires after a year, but the renewal is free and the renewal remains just as difficult to locate as the original licensing!

I'd suggest steering clear of Blender and Zbrush as it's too easy to create non-printable objects. SketchUp is the worst in that respect.

lol. Blender and Zbrush are my main goto software for 3d modeling. They are not easy programs to master but they are incredibly powerful for 3d modeling. Not for the timid (they are challenging to learn if you have not done 3d modeling before) or for anyone that doesn't have a few years to dive deep into them though you can pick up the basics in a few days/weeks.

These are the two main software I use for my cnc pattern making and now 3d printing model making.

Blender is a full on 3d suite. It can 3d model, digital sculpting, VFX, compositing, animation, sound editor, video editor, physics simulator, cloth simulator, water/fire simulator,...I am probably forgetting a bunch more.

Zbrush is the industry standard for digital sculpting.

Both maybe a little overkill for just adding text but they are a ton of fun.