If you’re going to split Pennsylvania, then it doesn’t make any sense to not split Kansas. Kansas City has very little in common with western Kansas, geographically or demographically - to speak nothing of of the fact that the majority of Kansas City is in Missouri.
Easterners think themselves either sophisticated if affluent, or savvy if working-class
Southerners think themselves culturally enriched and eccentric.
Westerners/Southwesterners think themselves rugged individualists
West Coasters think themselves cool and superior
Midwesterners think that anybody who cutivates any of those, or any other mystique for that matter, is a pathetic asshole.
Midwesterners deny having any discernable culture, there’s no corner of the world where a Midwesterner is viewed as exotic, and frankly, Midwesterners keep human civilization grounded so everyone else can indulge in their own nonsense. And they like it that way!
So it’s not really geographic; it’s more a state of mind. 
Neither Iowa nor Missouri is a Plains state–either culturally or ecologically.
Reading through this thread, the only conclusion I can draw is, “It beats the hell out of me”.
Midwestern United States - Wikipedia The Midwest.
Minnesota
Wisconsin
Iowa
Missouri
Michigan
Illinois
Indiana
Ohio
Those are the states in the Midwest. The Dakotas, Kansas, Nebraska, Oklahoma: these are the Plains states. Kentucky, Arkansas, and West Virgina start the South, Pennsylvania’s in the East.
The land area originally designated in the Northwest Ordinance of 1787. Everyplace else that claims a “Midwest” affiliation is just riding the coattails. Kinda like allowing Penn State and Nebraska into the Big Ten. Everyone wants to be part of the best.
Personally, I wouldn’t draw the boundaries along state lines: Cleveland, for example, is in the Midwest (though it’s near the edge), unless you carve out a separate region for the Great Lakes or something, but Athens, OH (towards the southeastern portion of the state) is Appalachia.
The Midwest is west of the Appalachians, east of the Great Plains, and has its southern boundary somewhere in the vicinity of the Ohio River (I’m not sure precisely on that one). Wisconsin, Illinois, Michigan, and Indiana are all entirely in the Midwest, plus most of Minnesota and Iowa, and part of Ohio. I’m not sure about Missouri, but most of that’s probably Midwest, too.
I think the wiki-map gonzo posted is probably what I would say.
Of course, the answer would depend entirely upon the complete question, which would have to identify the total list of categories. If the only options are East, South, Midwest, and West, I would describe the MW as wiki does. If you start adding in categories like Plains, West coast, SW, mountains, it can get trickier.
I tend to figure you can toss Kans/Nebr/Dak in with the Midwest because there really aren’t enough folk in those states for it to make that much of a difference. Tho the crops, stock, and rainfall are different from Ill/Iowa, the farming/ranching nature of their predominant industries makes them similar enough. One significant difference is the lack of big cities make them tend more Republican than Ill/Ohio/Mich…
There’s also Al Capone, right? Bang, bang!
I consider the Midwest of the Ohio, Indiana, Illinois, Minnesota, Wisconsin, Michigan and Missouri. North Dakota south to Oklahoma I see as the Plains states. West of that are the western states. I consider the Confederate states to be “the South” ('cept Texas) and have no idea how to classify Kentucky. Luckily, it doesn’t come up.
West of the Appalachians, north of the Ohio, east of the Mississippi, and south of the Great Lakes.
I’ve heard people refer to those from Buffalo as being from the midwest, but I’ve never bought into that.
I’m rather broad in my definition: The Midwest is that area to the west of the Appalachians, east of the Rockies, and north of Oklahoma. If your state includes the App. or Rockies, you’re not “Midwest”.
I think the central debate in this thread evolves from the geographic Midwest vs. the cultural Midwest.
The cultural Midwest might go as far east as Pittsburgh (and might be coterminous with the “rust belt”), but geographically the Midwest ends at the Ohio/Pennsylvania border. The states that are (in my opinion) definitely included in both would be Ohio, Indiana, Michigan, Wisconsin, Illinois and Minnesota.
Until you get to the coast. Pacific Northwesterners are sort of crunchy and outdoorsy while Southern Californians are all California-y.
I live in that area and I’ve never heard of anyone from here calling this the Midwest.
My wife said it once (she’s a Buffalo native). I called ‘bullshit’.
The only thing people with Buffalo have in common with people from Minneapolis is that they’re doomed to cheer for NFL teams that will never, ever, ever win the Super Bowl.
The only people who believe that are those who define the “East” as “places north of Philadelphia where you can actually see the ocean.”