PlantUML

PlantUML draws more diagram types than anything else available, and it has been doing it long enough that most of the syntax is stable and well documented.

It is heavier than Mermaid and worth it when you need a type Mermaid does not have, or the standard library shapes.

#What it is good at

UML in its full range, not just the four common types.
Diagrams that need the standard library: AWS, Azure, C4, ArchiMate icon sets.
Long-lived documentation, because the syntax rarely breaks between versions.

#What it draws

Diagram	Syntax starts with
Sequence	@startuml then participant
Class	@startuml then class
Activity	@startuml then start / :step;
Component	@startuml then component
State	@startuml then state
Use case	@startuml then actor / usecase
Deployment	@startuml then node
Object, timing, gantt	@startuml with the matching keywords

#A worked example

An activity diagram with a branch and a parallel section:

The source, which you can paste into a new diagram and edit:

@startuml
start
:Receive request;
if (Authenticated?) then (yes)
  fork
    :Write audit entry;
  fork again
    :Process request;
  end fork
  :Return 200;
else (no)
  :Return 401;
endif
stop
@enduml

#A few things that catch people out

Every diagram needs @startuml and @enduml, or it will not be recognised as PlantUML.
The standard library is included via !include; those shapes are what make C4 and cloud diagrams work.
PlantUML lays out its own diagrams, and fighting the layout is rarely worth the time.

draw.io

The canvas, when position carries meaning.

Excalidraw

The sketch that looks like a sketch.

Examples

Working diagrams you can copy.

Learn one properly. Borrow the rest when you need them.