The Galaxy Nexus has the most coherent, stylish and no-nonsense design (t's pretty much the only phone without freaking branding on the front!!). It's probably the one you'd be the least ashamed to use, as someone coming from an iPhone. At least that's how I feel - longtime iPhone user in the process of switching, still looking for the right phone. All other Android phones that I've checked out look more or less arbitrary in design, which is precisely what the iPhone does not. (The back of the thing is an exception to this, it doesn't match the stylish front at all - but this can be easily remedied with a slick lightweight case.)
It doesn't have a physical home button though, if that's a requirement for you, you'd probably want the S3. This one is terribly plasticky feeling though, and has heavily over-saturated colors in the screen. For me this is the anti-iPhone. It's so ugly I just couldn't bear being seen with one.
If the quality of the screen is of top importance for you, the HTC One X definitely comes closest to the iPhone screen of all Androids out there at the moment. Design and build quality are good one this one too, apart from the big letters saying "HTC" on the front, which makes me cringe just a little every time I look at one of these.
As for the OS experience, like people have said above, pure vanilla Android definitely comes closest to iOS. This is available on pretty much all phones with a bit of work, rooting etc. If you don't wanna get into that, the Galaxy Nexus is the phone for you.
Overall, the Nexus has the most similar feel to an iPhone. It's well designed, no branding on the front, and you get vanilla Android with quick updates. I find the screen a tiny bit fuzzy and if you're coming from an iPhone 4S, you'll hate the camera. But it still feels like the most coherent and sharp piece of kit of all these.
And like also said above, the next line of Nexus phones should be in the works and out soon, so best of all maybe to wait for one of those.