I've been a ham since 1986 but had not paid much attention to the various Over-The-Air digital modes until 2022, and then spent several months researching and experimenting with many of these.
AllStar is the highest quality, most reliable way to access distant repeaters and other nodes and users, hubs, bridges, etc. It is very easy to build your own node using a Raspberry Pi or Intel/AMD microPC, a node radio such as an HT or mobile radio, and the free and open-source AllStarLink software, which is easy to set up and has a large, helpful user community and great web portal, wiki and support forum. ASL has a huge feature set and in my opinion is the future of interlinked ham radio V/UHF repeater communications.
EchoLink is a great option for accessing repeaters, links or other users who do not have an AllStar node. Otherwise AllStar is generally a much better option.
D-STAR, like most other low bitrate digital modes discussed below, generally sounds robotic and unnatural. It can sound good if using a good quality Kenwood or Icom radio and a good quality microphone with mic gain properly set, but unfortunately most users do not have good audio quality. D-STAR is not user-friendly and definitely has a significant learning curve. There's no easy way to see how many users are on a reflector, thus there are 1,000's of REF, XRF, XLX, etc. reflectors that you could potentially talk on but 99% of them never have any traffic and there's no way to see which of them are active other than looking online for a "last heard" list which only works for some types of reflectors. To find reflectors that have some activity requires endless button pressing, by manually entering each character of the reflector name (eg. "R E F 0 3 0 C") in your D-STAR radio or node. There's no way to scan reflectors, or simply turn a knob like you would for analog. Thus it's no surprise that D-STAR adoption is relatively low and that the few repeaters that do exist are rarely used. If you happen to have some local repeaters that permanently link to a more commonly used reflector and/or have a lot of local activity that makes it easier, but in reality most D-STAR users probably never even look at other reflectors.
D-STAR may be fine if you have only 1 or 2 reflectors you want to use and if you're using a properly set up D-STAR radio. Otherwise you're likely to end up with bad audio quality, a primitive and tedious UI, and a weeks-long learning curve. However Icom has made some improvements over time and newer Icom radios such as the IC-9700 do sound better and are easier to use than older models. With a nice newer radio you should get better audio quality than with C4FM or DMR radios, but still not as good as Analog FM in most cases.
WIRES-X is similar to AllStar in that it allows nodes to be connected together using VOIP technology, but instead of being open-source, flexible, and extensible, it's proprietary and can only be used with Yaesu hardware. Fusion supports several C4FM OTA (Over-The-Air) modulation schemes, making it possible to use Analog FM, Data, Digital Wide Voice, or Digital Narrow Voice + Data. The data modes allow data and control information to be sent OTA, supporting menus on Yaesu radios to show a list of rooms along with a count of users in each room. You can then scroll through a list and click a room to enter - a vast improvement over D-STAR's UI.
As of 2022 a search on repeaterbook.com shows 48 D-STAR repeaters in California vs. 88 Fusion and 27 WIRES-X repeaters. However, of the 6 repeaters in San Diego County I tested that are listed as supporting WIRES-X, only 2 responded to WIRES-X commands. This would seem to indicate that the vast majority of YSF repeaters do not support WIRES-X and thus will only work for standalone local comms. These 'Fusion'-only (non-WIRES-X) repeaters are somewhat oxymoronic. I've heard very little activity on the dozen of these repeaters in San Diego County. These are promoted by Yaesu as being a great upgrade path since they support both Analog FM and C4FM, but this is little more than a marketing gimmick to sell Yaesu radios. For a repeater to remain 100% compatible with analog would require its output to always be in analog, with exception of WIRES-X control signaling which could also be sent if analog users enabled tone squelch, but because most Fusion repeaters don't support WIRES-X there would then be no benefit other than possible future expandability. And Yaesu seems to have made no provision for controlling WIRES-X functions through analog (ie. DTMF codes), which seems to confirm that Yaesu's intent is not to ensure compatibility with non-Yaesu radios. Fortunately AllStar sounds much better, fully supports analog FM and thus radios from all manufacturers.
The overall audio quality of YSF is about the same as D-STAR. I wouldn't invest in a Yaesu WIRES-X radio unless you have some nearby repeaters that fully support WIRES-X. Given that the 3 major manufacturers (Icom, Kenwood, & Yaesu) only support D-STAR or Fusion, both of which are proprietary and have limited adoption, my recommendation would be to buy a radio based on its overall features and performance relating to good old-fashioned FM. Having D-STAR or YSF could then be a useful feature if you frequently operate mobile and if you have a good number of nearby active repeaters supporting those modes. Testing I did with Yaesu's FT5D revealed that the audio quality is poor even in Voice-Wide mode and that the radio has many design weaknesses, bugs, poor receive sensitivity and short battery life. There are similar reports on their FTM-series mobile radios also thus it appears Yaesu stopped producing high-quality innovative HTs and mobiles many years ago, and their current products are little more than consumer-grade toys.
The overall audio quality of DMR is pretty low but in weak-signal/multipath conditions it can sound better than analog and as shown in this Test Video seems to also sound better than most other digital modes in difficult RF conditions. The available radios tend to be hard to program and not user-friendly. DMR is not recommended unless you frequently operate mobile, or have only specific talk groups you want to use that are not available on the other modes above. Most popular talk groups should have bridges to the other modes above thus you can usually access those TGs without needing to commit to a DMR radio.
M17 seems to be in early development, based on creating a digital mode using an open-source codec ("codec2"), which is great, however the audio quality appears to be no better than other codecs, with possible exception of at very very low bitrates, but at that point if you can only get ~700 bits/sec through I'd rather wait until I have better RF conditions, or use Analog FM/SSB. There seems to be very little if any use of the M17 reflectors, or any way to find out which ones have any activity. (It seems the trend with each new digital mode is a bunch of techies/developers jump on the bandwagon and create a reflector, 99% of which are then never used, leaving it to users to then have to sort through a giant list every time they want to connect somewhere. Like if a cable TV company only had 5 active channels but made you press buttons and enter in 1,000 random channel numbers just to find them.) Ideally the M17 team would provide an API enabling sorting the reflector list by number of active/recent users, and broaden the codec support and bitrate options to ensure the audio quality can be competitive with analog FM in the situations when you do have a half-decent RF connection and don't need to squeeze everything into 700bps. This seems beyond the scope of the project though and they will probably never support better codecs - such as Lyra which is also open-source and sounds much better than codec2.
P25 is a proprietary codec used by high-end Motorola and other commercial radios. From what I've heard the audio quality can be very good. As it is not available on affordable, easy-to-program radios however its use seems pretty limited, and there is almost zero activity on the P25 TalkGroups. Making things worse, many people bridge from other low-quality modes such as YSF, DMR, or various phone apps rather than from a high-quality P25 radio and thus all the usual glitches, klunks, dropouts and huge variations in audio levels end up making the whole thing pretty much unusable. If some day a manufacturer comes out with an easy to use, very high quality, real P25 ham radio product at a good price that might change things but it seems unlikely that will happen.