Jasper National Park for Christmas in November at Jasper Park Lodge. You didn’t say it had to be American… 
Smokey Mountain NP. Wife and I where on a long road trip and took a 3 night break in Gatlinburg. Did a bit of hiking.
For actual National Parks it’s either Acadia in 1993 or the Everglades in early 2003.
That more recent visit was a multiday guided kayak trip in the 10,000 Islands area. Part of that overlaps with area that is within the boundaries of the Everglades National Park. I don’t think we crossed into that area. Gulf Islands National Seashore in early 2014 is probably the most recent area administered by the National Park Service.
I tend to avoid the actual parks in the system. I prefer more wild, fewer amenities, and fewer visitors when I’m heading into nature.
I am in GCNP right now, do I win?
Zion NP in the spring. We’re headed back to Capitol Reef and Bryce in April.
Yellowstone, last summer. Simply stunning. I would go back again in a hot minute.
Technically, the OP’s answer isn’t a National Park - it’s a National Historic Site. So the answer to the question for most people really depends on what you allow.
For me, for true National Park - it’s Glacier Bay National Park and Preserve - almost 20 years ago.
Expanding the definition to all NPS locations, it’s probably the Lincoln Memorial National Monument, last July.
Really stretching it, I drive across the Appalachian Trail almost every day.
Assateague National Seashore, in September
Last National Park, strictly defined: Yellowstone, more than a decade ago.
Last park that is administered by the NPS: Chesapeake and Ohio Canal National Historical Park and Harpers Ferry National Historical Park, both about 18 months ago
Last land that is administered by the NPS but not designated as a park: Appalachian National Scenic Trail, in October.
Rouge National Urban Park, in Scarborough, ON. We go for a walk there at least once a year.
Badlands in South Dakota last August. On the same trip we were also at Mt. Rushmore and Wind Cave. My 7-year old granddaughter got Junior Ranger badges at all three of them.
One of the 500+ NPs in Australia. Let’s say Byfield, but it’s well possible we went through another one afterwards without being aware.
You win a fabulous prize! You can pick it up at Phantom Ranch anytime within the next 24 hours. 
Mt. Shasta, which is part of the Shasta-Trinity National forest.
Before that it was Yosemite.
Yellowstone with my family this past August
Smokey Mountain National Park. We approached from the southeast, climbed Clingman’s Dome, and drove north. (Avoiding Gatlinburg, but not all the other built-up touristy stuff). This was in September, just after Hurricane Florence, although the real goal was to see what the Smokies looked like post fire-- we didn’t see any burned buildings, but we could tell where the undergrowth looked different as a result of the fire a couple years ago.
Fords Theatre. I’ve been to 406/418 of the US park (NPS) units so far.
The National Mall in Washington, DC counts, right? Was just there last Monday.
Memorial Day week spent at Dinosaur National Monument, both states.
Denali National Park, this August.