In the early days it was ESRI for GIS solutions. Then Google maps, after Mapquest, then Yahoo, and uncle southern jakes crayola maps, and eddies cousins iroc gis….. on and on and on.
Good grief, to quote Charlie Brown. What the hell should I use for GIS? Well I can give you my opinion if your that concerned. Don’t hold your breath, like all advice… well actually hold your breath as it might just stink. All jokes aside, here goes.
If you are like me and enjoying writing Flex apps then here are the suggestions that I have for adding that GIS flavor to your application.
Google: Just showed up with a Flash (as they call it ) API, which means they wrote it in action script as an after thought. Probably because they realize Ajax is getting it’s ass kicked by Flex. Looks good, provides all the basic functionality you expect, now a day from a GIS api, but it still is lacking. For how long, who knows. Knowing Google they will bring it on strong and make a great Flex api.
Yahoo: Oh, the ugly cousin of Google. It works, does what it says, offers the same features, but… their dev team is about one person. Yes, I am still talking AS3 or Flex apis. They are not really that committed. It works, and the kid builiding it is really, really sharp. Now if Yahoo would just give the kid some help they would be all right.
MapQuest: Using it now for an application, enjoy it but the same thing applies. The same as the previous two. But one thing that is really glaring is the lack of community support. The docmentation sucks, the community is almost non-existent, and you are pretty much on your own. But, it does work and you just have to fight through the pain to make it work. Rumor has it that if you use this api and then purchase ESRI you can flip a switch and it will work with their software, if Universal Mind provides you with a tile server, but that is a rumor.. founded…. maybe… I don’t know 
ESRI: great api, super services and definetly the 600 pound gorilla in the GIS room. ESRI just needs to commit to making a decision if they will offer a Flex api with ArcGIS 9.3. Who knows what they will do…. I don’t think they have a clue either. Their api is solid, the gis tech is sick, but they are really confused right now.
In the end I would have to say that Google having an api available for Flex will kick everyone else in the pants. Thank you Google!
Just a GIS API RANT! Peace until the next line of code!