The problem in my eyes with comparing UIs is that it's so very much user dependent. I don't like looking at the iPhone UI, let alone using it (though lately I've started to get ridiculously in love with the command-line. There's like 10 file managers available for the N900 that have access to the entire file-system, and text editors to do any file editing necessary - yet I prefer combinations of cd, ls, and vi in terminal.)

So to be honest - the iPhone does UI smoothness and fluidity really well - I admit that. But UI as a whole? I just don't feel it. A better UI requires some standard to be compared by, but anything broad enough "usability", "intuitiveness", "convenience", etc, is linked to the user - usability is linked to user's uses (what they actually want/like to use their device for, and how); intuitiveness to the experiences of the user with previous interfaces they used, convenience to a mix of those and other factors (environment used in), and so on.

In practice, I think that the best UI is one that is flexible. It can start in a default configuration that prevents the average end user from getting in under-the-covers of the system, but it should be flexible enough to not only allow you to access controls or settings beyond what a completely unintelligent user would want to use for stuff to "just work", but also to let you radically change the actual structure or style of the UI.

As a limited example, as I understand it, the iPhone's UI has no desktop. It just has the existing rows or apps, which you can slide between. A closer to ideal UI would allow you to have the option of having desktops. Basically, like the N900 UI, but somewhere you have the option of just not having desktops, and instead have your homescreen be the menu, with the status bar appearing in that. (Then you'd have the option of determining whether you swipe up/down, left/right, either, etc, to scroll through the menu / app-list whatever.) I get that in practice this is more of a pain in the *** to code, and true complex customizing would require more willingness to fiddle than the typical person has, so you'd ultimately leave them with some basic options like above, and maybe a few possible over-arching UI 'paradigms' to switch between, each with a couple of options, but since we're talking about better vs. worse UIs.

But back to iPhone UI - I just don't like it. And I know others don't. So no, MohammadAG, I am not hating on you. I just think that UIs are too user-relative of a matter for any given UI to be called an advantage. I will concede that the iPhone UI is smooth, fluent, and reasonably usable. And that I can imagine other people liking it more than the N900 UI. But me personally, I know I enjoy the way the Maemo 5 Hildon UI feels more so than I ever enjoyed the iPhone's.