you can't compare the four phones (families) like this.
each comes with a different Operating System, in some cases (N8 / Symbian^3) barely visible as such, as the phone will ALWAYS start w/ Symbian, no matter what, whereas the N900 is properly a computer that might, in due time, be booted w/ other OSes (MeeGo, for a starter, but hopefully also Android / NT-Druid and other Linux distros). thus, in all fairness, what you ask is also a comparison of the four OSes each phone comes w/.
personally i know 3 of the 4 OSes (Symbian OS S60, Maemo 5 and iphone 3) and the HWs.
my absolute favorite goes to the N900 / Maemo, as it is indeed completely configurable and the HW properly is a mobile computer, with all its power (for a power user...) and all its twitches.
Nokia just brought Maemo (now MeeGo) to everybody's attention w/ the N900, just like they started Symbian OS S60 8 years ago with the 5760. chances are, in a few yrs nobody will remember Android or even iphones
the Symbian OS S60 is 2nd best, because even though the OS itself is not open to much tweaking @ least their is a plethora of (free) applications to improve your phone.
and the interface is simply the best around, as mobile phones are concerned. simple, straight, does what it is supposed to do.
Android; don't know, don't really care, but it is based on open source (partly) and there is plenty of ways to get a grip on the OS, if you care to; HW is offering a lot of choice / competition, so it is a matter of choosing the right device for you.
any thing left?
o, yeah, that piece of $h!t !phone.
don't buy it.
you only going to have to give Steve Jobs your cc nbr., so he can milk your bank account at the same time as your phone service provider. who would be stupid enough to do that?
and you can't even properly make a phone call w/ it >¦-))