Guessing this is a single DIN slot? I put a lot of Kenwoods eXcelon's in. Really like them. Put eXcelons in some applications and some of their similar units that are not eXcelon's but share a similar appearance and most specs and features with a lower price. The eXcelon KDC-X798 or the KDC-BT758HD  are the two I prefer. Unless you are putting in a huge component system with some serious sound, you don't really need the eXcelon. Most of the features it offers over the others, will not be noticed in a tractor cab. 

If the three you listed are what you want though, I would get the Delphi. REI is usually cheaper though if cost is more important to you. 

Warning: Long version/story below this point.

What I like?

• Value: For less than $200 you get a high quality feature packed stereo that is still simple to operate and not obnoxious. 
• Bluetooth: They come with a microphone you can mount wherever you please. You can calibrate the system for the environment you are in. Very nice for getting good quality sound to the people on the other end of the line. It still works great in tractor and semi cabs.  These radios also support A2DP which allows you to stream audio from your phone (music, youtube, etc) through the radio. I believe it also supports voice dialing and can store your contacts, but I never use those features on any radio. It does have a seperate mode with control for Pandora and iHeart radio through your phone as well. 
• Hookups: You can control an MP3 player/iPod through the USB, or put all of your tunes on a flash drive and just plug it in. There's also an auxillary port if you want. Both of these can be closed off by a small door. 
• CD Player: I cannot remember the last time I listened to a CD but that doesn't mean everyone has abandoned them. If you like CD's, these radios still have a CD player and the slot has a nice cover that keeps dirt out. 
• HD Radio: You would have to check your local listings, but in many areas there are quite a few HD channels. The HD1 channels are the regular channel, but broadcast in Digital (HD) instead of Analog.  In a good environment you can really notice the sound quality difference, but in a loud tractor not so much. No static though.  The HD2, HD3, and HD4 channels are all additional programming that are different from your regular FM channels. I myself do not listen to much radio anymore, but its nice having more options when I do choose to. 
• Remote: This is something I really like and part of why I swapped one of these into our Fendt right away. Some tractors, like Fendt, have nothing to rest your fingers on to brace your hand when trying to navigate the radio in a rough field. Newer Magnums have the little ledge. Newer MF's have a little bar. The Genesis tractors were still my favorite with the big handle. I think the NH speedrower we used to have had one as well.  The remote makes life easier especially with the tiny buttons on these radios (one of the downsides). If you have a lot of people running it, I suggest mounting the remote to a gear keeper so that it is always suspended close by when needed, but cannot be dropped, lost or smashed on accident. 
• Personalization: Very minor, but to some people its a nice touch. Being able to choose the color you want for the lighting, pick your display layout, etc. 

Dislikes

• Glare: The screen is hard to read in daylight. Some anti-glare film can help.
• Buttons:  The buttons are a bit small. If you want to test out the feel, find a local audio store and go check out their displays. You can also jump up to the next two models which use the knob to scroll through the presets instead of pushing little preset buttons. I get the ones with little preset buttons because too many people can't figure out the dial, but I think the dial is nicer to use, plus you can store a lot more presets in a row than you would be able to otherwise. Especially nice with the extra HD stations, you can scroll through all of your stations without having to change to the next set of 6.

Keep in mind, the above things that I mentioned I like are not necessarily specific to these radios really in any way. You can find these features on many different radios.  Finding "tractor style" "heavy duty" radios with all of the above features is not easy though and often gets very expensive. Personally I feel heavy duty radios are overrated. They are not that special and they still fail. Even if they are "tougher", they are drastically overpriced.

Installation of one of these will not be "plug and play", but will ber very easy. This applies for any single DIN radio. You will remove the old radio, discard the old "cage mount" if applicable. Then install the cage mount for your specific radio by sliding it into the hole. To hold it in place simply bend the proper tapered tabs over. It should be nice a snug when done. The radio will simple slide into this mount and lock into place. To remove it, you will need the removal tools for the radio so tape them in a glove box or something so they don't get lost. If you do loose them they are not hard to come by. Some radios like the Kenwoods will have a trim piece to snap on that hides the slots for the release tools.   The toughest part will be wiring.  The new radio will have a diagram but the old radio may not.  With some research you should be able to figure out all of the wires. Then you can either splice the wires, or put them into a connector. Your choice. There is not a bagillion wires back there. You need a constant 12v, keyed 12v and ground for most all radios. Some will have a dimmer wire, some will have other wires that may or may not be used, but those are the basics. You will then have a + and - wire for each speaker that you have.  Its pretty simple and basic wiring.  Often times you can buy a plug for your vehicle/tractor and simply splice the appropriate wires from that plug with the ones from the radio plug.  Much easier to do it this way on a bench than crammed up in the radio slot in the cab. 

Here are some photos and Amazon links(for simplicity, plenty of other places to buy).  If you are browsing and comparing audio stuff, I suggest going to Crutchfields site. Its very well laid out with more detail than some of the OEM's. You will notice there is not much difference in appearance between the eXcelon and the non eXcelon. 


This is the eXcelon KDC-X798 

Buy on Amazon



 



This is the KDC-BT758HD

Buy On Amazon


 




This is the eXcelon KDC-X998 which is two steps up from the X798. Compare the features to decide. No preset buttons and using the Knob is a big change you may or may not like.

Buy On Amazon





This is the KDC-BT958HD which is two steps up from the BT758HD. Compare the features to decide. No preset buttons and using the Knob is a big change you may or may not like.

Buy On Amazon

 






This is the remote that all of them come with. 



 


This is the small Microphone (included with all BT models) for the Hands free Bluetooth calling. It has a small visor clip on it. You can also opt to velcro it to the headliner somewhere. 



Good Luck.

REI is as good as anything, if you just want a basic radio that is easy to install and use.

The only one you mention I have experience with is REI.  I wouldn't buy one again.  I put a REI model ST-1000 in a tractor 2 years ago and it is already dying.  It functions, but it occasionally changes volume spontaneously when hitting a bump.  It actually thinks the knob is turning as the volume indicator on the screen goes down.  Also, sometimes when turning the knob up by hand the number goes down, then up.  Really poor stuff in my opinion.

My go-to radio for farm equipment is this one:

http://www.crutchfield.com/p_522JHD1130/Jensen-Heavy-Duty-JHD1130B.html

I've got 3 of them in various machines and they have all been entirely trouble free. Even put one in my farm pickup.