This Indiana license plate in the mid-90s was a thing of beauty. Indiana’s plates are very boring now.
Indiana holds the record for the most consecutive years with mortifying plates, and I can remember them back to the 40s.
One of Idaho’s biggest products is rapeseed oil. Some years ago somebody wanted to have their license plate motto read “Idaho - The Land of Rape”. Didn’t go over too good.
Maybe I’m a little too 20th century here, but I like the California Whale Tail plate.
It has a bit of messy history too. The state got into a dispute with the artist who drew the original design, and eventually the state just dropped that design and went with a different picture. The newer one is nicer.
Article discussing it, with both pictures, here:
Dispute Sparks California License Plate Redesign, July 2011.
The upper picture appears to be the newer one, according to the descriptions in the text. I like it better.
Probably the Patriots and Red Sox specialty plates you can get in Massachusetts, since they are my favorite teams. I also like the Vermont conservation plates Specialty Plates | Department of Motor Vehicles although like the standard plates the green background can clash with many car colors.
It is time for the states to raise big bucks by selling vanity European-format plates. Charge BMW people (say) $5,000 a pop and rake in the dough.
I really liked the old TN plates that were shaped like the state. See the right-most plate in the second row of https://www.google.com/search?q=traditional+tenn+license+plates&tbm=isch&source=univ&client=firefox-b-d&sa=X&ved=2ahUKEwiw5ZjUl-joAhXGVN8KHQ3WAkMQsAR6BAgKEAE&biw=1491&bih=750. PA used to have the outline of the state on a standard rectangular plate. Of course they hit bottom with “You have a friend in” plates.
Missouri’s Cave State specialty plate is pretty nice. I’ve never seen one on the road though.
I had a room mate from Indiana in the late 80s. His plate simply said ‘Wander’. Ugly as hell.
I agree that Texas plates are dull as dishwater (closely rivalled by Ohio plates which look monochrome dirty white but on very close inspection are crammed with tiny pale gray lettering detailing all the wondrous things to be found in Ohio.
What gets me is what “safety” has to do with it. Are cops and license plate readers only able to identify plates that are white or very pale with no design to speak of? Really?
When I am elected governor one of my top priorities will be to have a contest to design a really cool standard license plate. In color.*
*when I moved to Kentucky I was tempted to get this plate but was too cheap to pay extra for it.
If you like simplicity Iowa’s “Black Out” plates look nice. They sold out when first offered and I wouldn’t mind if it was Iowa’s “standard” plate.
Another vote for New Mexico and Colorado for nice plates. Hawaii had pretty ones when we were in school here in the early '90s, but they’ve turned boring since.
Apparently the new white-letters-on-blue plates in Ontario have had some readability issues.
“Coal keeps the lights on”? Not in Ontario… (Scroll down to “Electricity” under “Energy Production”.) 
The 1980s Kansas plates with the wheat have always been one of my favorites. Kansas License Plates - 1980 "Wheat" Plate
In the 30’s, some South Carolina governor told his aides to find something in which the state leads the nation. Finally, they found one : industrial Iodine. So for a few years, the word “Iodine” appeared on the plates.
https://www.google.com/imgres?imgurl=https%3A%2F%2Fsaleofcar.com%2Fimg%2F1933-south-carolina-license-plate-231841586600%2F0.jpg&imgrefurl=https%3A%2F%2Fsaleofcar.com%2Fplates%2F1933-south-058473&tbnid=dY-7WSy5Gqf5yM&vet=12ahUKEwjNncbgsenoAhUM4JQKHalYCu0QMygGegUIARDSAQ..i&docid=GdSD_0-PD1fb8M&w=726&h=545&q=south%20carolina%20plates%20iodine&ved=2ahUKEwjNncbgsenoAhUM4JQKHalYCu0QMygGegUIARDSAQ
When Michigan was finally forced to give up its beloved blue plate, it was explained an interstate compact required that the whole plate had to be safety-reflective, which required a light background.
Those are two good points I was going to make and got ninja’d on.
The thing about Ontario plates is that they’ve always been boring – not ugly, just boring. The recent change introduced by the new Ontario government improved on that in the following way: now they are not only just as boring as before, but also almost unreadable at night, according to police and several safety agencies. So you guys can go on about your pretty or ugly license plates, as the case may be, but we have the ultimate: invisible license plates! 
Apparently all the new dark blue ones are being recalled.