As an N900 owner, I have to say that you don't want an N900 at this point unless your a hacker or you need terminal and a bittorrent client on your device.
You should also look at devices like the Galaxy S phones, (Samsung Vibrant, Samsung Captivate, Samsung Fascinate, and the Epic 4G), the G2 which you can preorder anywhere now and the Evo 4G.
Like Dave said give us some more info and we can point you in the right direction.
Hope this helps since I have 3/4 phones that you have mentioned (no droidX since its cdma).
I've owned the n900 since December last year and is maybe my favorite of the bunch. The freedoms it offers can not be matched by the other two platforms.
But as dmpt put it, its too late in the game to be buying it unless you are a real enthusiast.
Besides, the n900 has a lot of drawbacks. Its big, its clunky, the build quality is not very good (my wifi's been busted since april & Nokia cant fix it! + i know two other friends who have had the usb connections come out or screens crack), the battery life is terrible etc.
I used the iphone4 for 3 weeks and you have to hold this thing to appreciate it. It looks and feels really nice in the hand and can be "fun" to use. The app store can add a lot of value if you know what to look for apart from games.
However I travel a lot and often times the gsm signals are not very strong. Being in the habit of holding the phone in my right hand with my index finger the antenna would get covered and calls would start cutting off. For a phone thats just not acceptable.
This is a personal opinion but iOS has always seemed to be too dumbed down to me and especially after coming from an n900 its mind boggling how many compromises one has to make to use this thing in daily life. Also the iphone 4 is CRAZY expensive in Asia. I've seen it at 900$ us (via a friend in HK) to 1800$ @ Dubai airport to 2200$ in Oman! I know the last two might have been people trying to rip me off but $1000 for a piece of tech that will be "old" in less than 12 months is crazy!
Been using a Nexus one since June and have mixed feelings. While i'm in the US its the phone to beat with all the Google integration. Abroad it can be a nightmare.
In Asia (& ME) you wont have access to the proper market place which means you wont get access to any paid content or stuff that is US only and unlike Apple thats most of the stuff! (I have not even been able to get the gmail update!).
The media playing abilities are anemic to put it mildly, the phone reception is not as good as nokia's, it requires you to be connected to the internet ALL the time! In most countries I've found that the phone does not recognize the sims APN's automatically, so you have to add them manually which can be a headache. In the cases that data does work you probably dont want it to since data rates in the region can be ridiculously expensive and if left to its devices, Android would run up your bill in no time. This can be a major sore since most of the applications refuse to run unless you are connected to the internet.
All 3 platforms have their quirks. The best advice I can give is try and hold the phones in your hand (go to the shop, play with one if your friend has it) and spend some time with them doing the kind of stuff you normally do. If you like the way it works then go for it :).
If you have any questions, we'll be glad to answer as best as we can!
Thanks guys for alll your help but after searching i found that the best phone that i can get this days that has great performance and a cool OS , very flexible is the " HTC Desire ". any body have any comment on this phone just say it before i get the phone
The US has a uniquely insane cellphone market and an HTC Desire is not worth using here, (It won't have 3G, it will be hard to fix if it breaks, you will be on your as far as firmware updates.) In the UK or in other parts of the world this is not a concern.
What do you see your self doing with the phone?
Are you a photography geek who wants the best cameraphone in the market? Are you a music lover who wants perks lake stereo speakers, mobile playlist creation or *shutter* iTunes integration. Do you want to use your phone for turn by turn navigation, portable movie playing, exchange support and physical keyboard to pound out work email, and/or internet tethering (Wi-fi, USB or Bluetooth).
The Desire fairs well over the above but its not tops in any of these categories.
Are you willing to pay for unlocked phones or switch carriers?
There might be more money involved in these cases. If you live in the US you would end up buying an unlocked HTC Desire and choosing a T-Mobile or AT&T plan. I'm not even sure if AT&T sells just SIM cards so you might end up with a useless phone.
The Desire is essentially the same as several devices in the US market (again assuming you live in there).
The Nexus One has stock Android instead of HTC sense and has a trackball and is best Desire substitute on T-Mobile or AT&T. T-Mobile also has the G2 which is so simlar to a Desire with a keyboard that its the Desire Z outside of the US. (Which I guess makes it closer to a Nexus One with a keyboard all things considered)
Verizon has the Droid Incredible which is the same phone as the Desire except it can't do voice and data at the same time due to its 3G technology.
The Evo 4G is the most similar phone on Sprint with the advantages of 4G and HDMI out and the disadvantage of a bigger screen (if you regard the desire as the perfect size) and an LCD screen. The international doppelgänger of the Evo 4G is the Desire HD.
This is a fantastic and brilliant breakdown on things to consider when purchasing a new cell phone. Bravo! I think this would even be worthy of its own post.
That's completely silly. I have both an Android 2.2 device and an N900 (1.3) and the Android device is not easier to use. The only time the n900 isn't as nice is it doesn't have hardware (pick up/hang up) buttons. Outside of that it's a much nicer OS to use. It's more consistent and less confusing.