Personally I'd dig up that old Churchill quote and say that Windows Mobile is the worst smartphone OS except for all of the others.
iPhone's slick, modern, and powerful, but is missing a lot of features that other platforms have had for years (even in OS 3.0), is incredibly locked-down yet very easy to crack (taking away most of the stability / security / anti-piracy benefits to its being so locked down), requires one to tithe 30% of one's revenues to its creator and comes from a company with a pretty terrible track record in general as far as how it treats developers.
Symbian to me at least seems just as buggy / slow as Windows Mobile, but is harder both to program for and to use (I've complained here in the past about how long it took me to set up WiFi for the first time on a P1i), and given the direction they're moving it could easily end up as locked-down as iPhone soon.
Android aside from what I consider the unpardonable sin of being Java-only is woefully immature at this point (c.f. Sparkletron's comments in the Android thread), and requires you to ignore the fact that it's probably not in Google's long-term corporate interests to encourage a healthy market in "offline" software for it or any other platform.
webOS looks promising but won't be able to run Pleco or any other "serious" software until it adds support for native development (which could be years away), and BlackBerry can't run any "serious" software even now owing to its lackluster programming environment. (and commits that same Java-only sin as Android)
Windows Mobile, for all its quirkiness and instability, is easy to program for, easy to use, more open / customizable than any other widely-used smartphone platform, and comes from a company which for all its faults has been downright heroic at times in its efforts to help and support developers - nobody puts as much effort into API design or backwards-compatibility as Microsoft, or gives developers access to its future product plans as completely and as far in advance. It's handicapped by its reliance on the somewhat-creaky Windows CE, but if you believe Steve Ballmer's recent comments they're eventually going to merge it with (or at least link it much more closely to) desktop Windows, and combining the openness and ease-of-use of Windows Mobile with the speed and stability of the Windows NT kernel would make for a really formidable mobile OS.