These should all get escaped:

Backslash: \

Backtick: `

Asterisk: *

Underscore: _

Left brace: {

Right brace: }

Left bracket: [

Right bracket: ]

Left paren: (

Right paren: )

Hash: #

Period: .

Bang: !

Plus: +

Minus: -

These should not, because they occur within a code block:

Backslash: \\

Backtick: \`

Asterisk: \*

Underscore: \_

Left brace: \{

Right brace: \}

Left bracket: \[

Right bracket: \]

Left paren: \(

Right paren: \)

Greater-than: \>

Hash: \#

Period: \.

Bang: \!

Plus: \+

Minus: \-

Nor should these, which occur in code spans:

Backslash: \\

Backtick: \`

Asterisk: \*

Underscore: \_

Left brace: \{

Right brace: \}

Left bracket: \[

Right bracket: \]

Left paren: \(

Right paren: \)

Greater-than: \>

Hash: \#

Period: \.

Bang: \!

Plus: \+

Minus: \-