I take it by Farnham, you mean Farnham Royal? (Cos the other Farnham is down near Fleet and miiiles away)
It depends upon how nice and area you want, your budget and house requirements, and your need for easy train access.
Farnham Royal is a very nice area (though to the very south of it, too far and you hit Britwell, a huge and grim estate on the north edge of Slough.) Farnham royal is Bucks so like Iver, if you pass the Bucks exam you will get a place, if not, then I think the local non grammar is Chalfont Community College (general thought is it's good but absolutely enormous.) there's no direct train access unless you drive to Burnham or up to Gerards cross/Chalfont.
Iver is nice, if you can afford Richings Park it is near what will be a Crossrail station, there are buses to Chalfont, Burnham and the Khalsa sikh school in Stoke Poges. Again, kids usually go to Chalfont.
Langley is less "posh" than Iver/Richings Park/Farnham Royal BUT it's Crossrail, you get more for your money in terms of house size, it's Berkshire rather than Bucks, but you can still do both eleven plus exams, and if yoi are SL3 7 or SL3 8 postcode you get priority cachement for both the Upton Court and Langley Slough grammars.
Can you tell I've been looking into this recently for my ten year old? 