As fate would have it, I’ve got one of each of these three. These are subjective personal reactions to them, but... The Keen is the budget entry and it has a nice smooth action but falls the shortest of the three when it comes to filling my XXXL hand. The Hills also has a nice smooth action, fills my hand the most of the three and a slightly longer blade. The Meg has a really smooth action, falls in the middle when it comes to filling my hand (which it still does quite nicely and very comfortably) and has the longest blade.
For the blade and handle sizes, the Meg is suprisingly lightweight and it feels and carries very nicely. The Hills looks, feels and carries clunky in comparison and the Keen looks, feels and carries what I’d call a bit more delicate. In terms of “pocket pecker size”--which some people get all bent out of shape about--the Meg has the smallest one, the Keen’s is only just barely bigger than the Meg’s and the one on the Hills is really big. I’d rate all three as being roughly equal in terms of “drop shut” action. I haven’t done any sharpening, disassembly, cleaning or maintenance on any of them so I have no insights into those areas.
The Keen and Hills both have S35VN blades but the Meg is M390, which means a great deal to a lot of people.
The Hills was one of the earliest Reates and is out of production, but it’s the last >$200 Reate (and I think the last >$350 Reate) you’ll ever get to grab. The Keen made a big splash when MD dropped it and it has a large, vocal community of lovers here @ BF. I’m sure they’ll be chiming in shortly. I don’t think I’ve seen the Meg get a lot of traction or develop any real kind of following here, and I honestly don’t know why. IMO it should have. I see it as a smoother version of the ZT 0452 with a blade made of better steel (than the standard 0452).
If you have XL to XXXL hands, I think you might find the Keen a bit more finicky to handle. I’m in the XXXL category and I find keeping my big meaty sausage fingers off of the lockbar when I go to deploy it to be an issue. I can work it OK, but it demands more careful finger placement for me.
All this being said--and being an XXXL hand size guy--I like the Meg the best and it’s my favorite of the three. The handle size is for me a nice compromise between the other two, the action is so smooth and easy that it deserves to be called “buttery” and I like that big damn blade. It's what many would probably call a pocket sword so if you don’t want a big blade it may not be best for you. If I had to pick just one out of the three, I’d unhesitatingly go with the Meg. The Hills would probably be my second choice and the only reason the Keen would be in last place is because of the care I have to take with finger placement when I deploy it. The Keen is the cheapest @ ~$140 and the Meg is the most expensive @ ~$210, but the Meg is still my favorite.
Edit: I also have a MD Gent, but it’s vastly smaller and in an entirely different category of knives than the three you asked about. It’s a nice little gentleman’s knife, but comparing it to the three you asked about would be like comparing dogs to horses.