We've been through these comparisons a couple of times. There are two main aspects: how good they are in stock form, and how much can they improve with minimal changes (ear pads and cables).
A very short recap IMHO.
In stock form, the D5200 might be the best buy, you don't have to mod them.
The D7200 in stock form is the best measuring closed headphone. Some D7200 were prone to midrange honkiness, but it could be fixed with ear pad changes. The apparent lack of treble is relative, the measurements show quite linear treble, but all other headphones tend to be bright in a way or the other (by measurements). After some ear pad optimizations (cut ~30% off the internal foam) the D7200 sounded better then a stock D9200. Some people still find the treble lacking, but I surmise they just might became used with other, bright headphones. Newer (1-2 years old) D7200 are reported to have improved on these accounts.
The D9200 are close to reference sounding headphones (open or closed). They have a slight brightness tendency around 10 kHz, it comes from the driver and it's hard to fix by other means then direct damping (thicker dust cloth) which will affect the rest of the spectrum as well.
When used with optimized pads (the same ones as with the D7200), the D9200 has better bass and midrange resolution, it is not a small difference. You hear more differentiated bass lines (acoustic instruments and electronic alike). The price for that increase is some treble hotness, which I am fine with, but others are not. The overall sound quality is closer to electrostatic headphones, but with better bass slam and depth (in most cases).
The TH900 are peculiar headphones. Their coloration is specific, a kind of W shape, but sound louder than either Denon, are brighter (around 9K) than either Denon, and have bigger sound stage and bass slam. In stock form they sound a bit honky IMHO. They need pad changes and eventually some different cups damping. In an optimized form, they sound acceptably linear, with a big-space bass slam (good for movies and games), centered around 40 Hz. The D9200 have a deeper thump (25-30 Hz), but dryer than the TH900, also more linear in the bass, and have better bass resolution. In the midrange, they have similar resolution, the TH900 is more laid back, but the D9200 has more complete harmonic structure. The TH900 coloration results in a pleasant, but kind of equalized sound.
I think the TH900 responds best to EQ, but all Denons fare also well. You will be fine with either of these headphones.
In stock form, all Denons have a smaller, more intimate sound stage, the TH900 sounds bigger. But those colorations were too much for me.
Note: I still have all these headphones, and since their utility value is far higher than their market value, I am keeping them (distributed among kids).
Today there are a lot of other options as well. One (not very good, but comprehensive) comparison video is
here.