They are all nice.
Of the 3 Swolphin hotels, my favorite is Swan Reserve, then Swan, then Dolphin. The SR is a slightly longer walk, but the hotel is very nice. I am slightly spoiled though as several of my stays were in suite rooms.
If you are an early riser/want to make early entry, a big plus of SR/S/D hotels is that is it is possible to have your breakfast or coffee very early. Here's the deal: early entry at AK starts at 7:30am. Breakfast at many WDW hotels doesn't start until 7am. (YC is 7am, though Gasparilla at GF opens 6am, Bwk Deli 7am, BC 7am, Fuel at Dolphin 6am, java at Swan 6am, grounds SR 6am). Not only do the S/SR/D have breakfast at 6am, they also offer early room service. I don't often choose to do room service, but it is very helpful/big time saver on AK mornings.
A downside of the S/D/SR is the bus service to MK. the buses now drop off at the TTC, or at least they did the last time I checked. Personally, I usually drive to MK/AK, so this is not much of an issue. From any of the resorts you list it is easy to walk to both Epcot and HS. It is also pretty easy to take the shuttle boats, but we can usually walk faster than the boats arrive. At any of the above, the boat dock will always be some walking distance from your room.
No matter which you choose, you can easily eat at any of the dining in that area. It is also pretty easy to get to most of the WS dining, except you might need a park hopper ticket (if you want to eat there on days you plan to visit MK/AK/HS).
S/D/SR charge a parking/resort fee. When looking at hotel rates on the Marriott website, make sure you check the box to see rates with fees included. The Marriott owned properties do offer discounts for: Annual passholders (code QWH), gov't employees/military, Seniors, AAA, nurses/Dr's/Paramedics/Firefighters, Teachers. If you qualify for any of the above, do check. (if you pay to park at S/D/SR you will then get free parking at the WDW parks).
ALSO- just as WDW releases various discounts, the Marriott rates are now also highly variable. (dynamic pricing) the rate you are offered today might go up or down tomorrow or next week.
I would NEVER book a non-refundable rate!!! Also, try playing around with your dates. Tweaking your dates by a day or two can often = a lower rate. Also, consider checking back on rates after you book there, because rates do go up and down. If you don't have AAA, and don't qualify for any other discounts, consider getting it. I haven't checked in a bit, but AAA used to also get 20% off the Marriott restaurants. sometimes the Marriotts also offer a AAA Hot Deal rate that is lower and refundable. You can also use/earn Marriott points if you are a member.
Another quirk to know: YC and the S/DSR all give you access to the PM extra hours, usually Mondays = 2 extra hours in Epcot, and Wed= 2 extra hours in MK, but that can change some weeks. The WDW owned hotels tend to have higher rates Thur(+)/Fri(++)/Say(++), and tend to be discounted Sun-Thur. The Marriott hotels are sometimes higher on Mon-Wednesday than weekends, but it varies. I think pricing is variable based on bookings.
You might encounter conventions and/or weddings/events at any of the above.
The pool at YC is fabulous, but the water slide and parts of the pool are too deep/fast-flowing for small children. On the other hand the pool has a beach/tidepool area where small children can play in the sand. the feature pool for the Marriot hotels is also very nice. Better than many of the WDW hotels. It is an attractive grotto, plus two Olympic size pools with lap lanes. Few WDW hotels have pools that are good for lap lane swimming.
The beds are great at all of the above. Once upon a time, regular rooms at Dolphin were full not queen, but I think they are all queens now. I'm not sure. When I've stayed at SR, I had a suite. Compared to WDW, it was a small upcharge at the time. Suites at YC are pretty crazy $$$.