Which punctuation mark would best fill in the blanks in the following sentence in the past year I visited Denver, Colorado Portland Oregon and Seattle Washington

Semicolon.

Explanation:

Although list items are commonly separated using commas, when the items themselves contain commas, it is possible to use semicolons instead to outrank those commas. This is necessary in this example in order to discern between cities and their respective states.

The correct sentence would be:

"In the past year I visited Denver, Colorado; Portland, Oregon; and Seattle, Washington."

In the past year I visited: Denver, Colorado, Portland, Oregon and Seattle, Washington.

In this case I used a colon, and commas.

The colon is used to connect two independent clauses when the second explains or illustrates the first.