Well, let's see.

Here's a pair that are both Venetians, I'd say...

"low" -- Bembo
"significantly higher" -- Iowan Old Style

BTW, the best way to look at x-height is to compare the lowercase with a capital. Extenders can always be lengthened. Look at the Trinité variants, for example. Or Seria and Scala, though the two are a little bit different in other ways.

Here's my take on (serifed) fonts with a high x-heigth: You can't properly kern the r, u, w and y with a cap T.

@Kristen Dudek: What are the lowest and highest you have found during your research?

Of course, there are all those ITC big x-height versions of classics, which were designed in the 1970s and 80s. Most contentious was the Garamond. Also ITC Cheltenham and ITC Caslon.

In the same era, Headliners International (franchised typositor type shops) produced big-x versions of just about every previous type, which they termed “Neo Mini-Whatever”.

Les Usherwood (Typsettra) produced many large-x designs, eg Flange. Again, same era.

Koch’s Antiqua, aka Eve, very small x.

Antique Olive, very large x, and small extenders.

Advertisers Gothic, early 20th century, big x and very short descenders.

Check out the FontBook.