The choices are:

Samsung Galaxy S2
Samsung Galaxy Nexus
Sony Experia S
Blackberry Bold 9900

I really want the HTC One X but not willing to wait because I haven't had a mobile since August last year.

Bound to get the fanboys going this one

S2 - good solid phone. low(er) screen resolution. Due an updated version within weeks/months. Decent Camera. Can get some good deals on it at the moment. Unlikely to get official updates beyond ICS.

Nexus - (my personal choice) big screen (some think too big). High res screen (less zooming to read webpages). 'Instant' camera (but lower mp than the S2). NFC (if that ever takes off in the UK). Vanilla Android so guaranteed updates for quite some time. BUT no SD card slot.

Sony - only experience with Sony is my wifes Experia Arc S which looks to be very sturdy and a quality piece of kit. The Sony bloatware doesnt seem as bad as some either. Sony also seem to be doing the right things with unlocking bootloaders etc - which is good.

Blackberry - whole different kettle of fish. Used to be 'the one' for emails but now most other OS's have caught up. The only reason I (personally) can see someone going for a Blackberry these days is if all of their 'mates' use BBM (which seems to be very popular in the 12-16 age group !). Fewer apps .... bit of a niche product really.

But ... nobody can tell you what the right phone for you is. Go into carphone warehouse or phones4u. Try the look and feel of each of your options. Everyones opinion will differ.

There is no 'Best' phone (despite what you will hear lots of posters claiming !)

Its a pity you cant wait for another couple of months. There have been some great phones to be released. The phone you get will be out of date in a few months time. In July the Samsung Galaxy S3 will be released. Going by the rumours, its worth waiting the extra months for it.

Also depends on your budget I would say. I am getting an upgrade to the S2 tomorrow as I have a family rate with T Mobile and get the contract half price. That makes it affordable.

If I waited for the S3, I suspect the contract would be that much higher and even half price, it would be a bit much. For example, the new iPhone is above my limit even at half price.

That also ties up with how much of a gadget freak you are! If having the very latest technology means everything, then wait for the next wave of phones. If like me, the phones you listed have pretty much everything you need already, go for it now. With the amount of apps on the S2 for instance, you can keep updating the phone as you go along anyway.

^^ good advice. go for what you can afford. NONE OF THE ABOVE ARE BAD PHONES !

Thanks for all the comments.

My budget is £600 and my preference is Samsung.
I think I'll have to go into the shop and try them out and make my mind up then but at the moment I'm inclined to go for the Nexus.

OK. Pros and Cons for the Nexus (from a Nexus owner)

Pros
Screen is fantastic ! very crisp and clear and a very good size.
Developer support is great.
Regular updates from Google
No bloatware
Great sound quality (via headphones)
Great look and feel (in my opinion looks very classy)
designed specifically for ICS (no hard menu buttons etc)
great GPS lock (using barometer)
size is perfect (for me). slim to fit in your pocket whilst still having a large screen.
Fast camera (dont believe the bad reviews - you need to learn to focus to get the good results !) - instant shutter ! Its no SLR but for snapshots its great.

Cons
No SD card slot (but im learning to live without it)
The size (a pro for me but a con for others)
The screen isnt gorilla glass (and the 'equivalent' used isnt as tough - buy a protector)

definitely try one in a shop before you buy to check that the size doesnt put you off. If you like tweaking and flashing ROMs then the Nexus is definitely the one for you

I went for the Samsung galaxy S2.
It was delivered today and I absolutely love it.

getting a new phone delivered is one of life's great buzzes

I agree with you on that!
I already had the case, protector and extended battery ready and waiting.

Still got a month or so and it is toss up between the new Sony xperia s, galaxy 2 or 3 and HTC one. The one is the one(no pun intended) I realy want now just depends on the reviews and cost. I think the galaxy 3 is going to he even bigger than the last one. That is too big for pockets.

Blackberries still seem get pretty good international options for those who travel a lot compared to regular phones. Certainly the case on T-Mobile at least.