This is more often than not the first, and most obvious, aspect that people think about when imagining "ideal". The adage says that "beauty is only skin deep", and this implies that personality etcetera are deeper and more important. However, there's a lot more to beauty, or physical attraction, than simply being "in the eye of the beholder".
Present-day humans are the result of billions of years of evolution of life on Earth, and as a result many factors determine our success and advancement as a species. Perhaps the most important of these is our ability to advance our species through positive procreation. By this I mean that it is in our genetics to choose the best possible mate for reproduction in order to create a better version of ourselves with each generation. While we may think we are in control of the choices we make in this area, these choices are actually so deep-rooted in our DNA that usually we are completely unaware of what we find attractive and why.
In principle we are subconsciously led by our need to create the best possible offspring that we can, and more often than not this means finding a mate whose strengths balance our weaknesses, and whose strengths strengthen our strengths. We want healthy children with the best possible chance of finding the best possible mate for themselves in a highly competitive "survival of the fittest" world, and for this we are drawn towards someone who is physically fit, has a healthy body, healthy-looking hair and an attractive face, as this will normally help produce good quality progeny. But exactly what is "attractive"?
Scientific research has shown that our brains are programmed to recognise beauty and perfection in all things, and physical beauty appears to follow a mathematic equation known as the 'Golden Ratio'. This equation was first described around 2,400 years ago, and appears extensively throughout nature, as well as art, architecture, music and even financial analysis. Put simply, it states that the ratio of 1:1.618 can be used to measure aesthetic perfection or "beauty". The ratio works on a human face, mathematically, by measuring one feature's juxtaposition to another, and by measuring a feature's length in proportion to its height. By superimposing a 'grid' based on this ratio over the face of someone considered to be beautiful (or otherwise), we can see how closely their features match the grid.
Good examples of people we consider beautiful are Megan Fox, Ava Gardner, Marilyn Monroe, Jessica Simpson and Angelina Jolie. Ms Jolie's "bee-stung" lips can be seen to significantly extend outside the grid, which does not strictly adhere to the ratio, but pretty much everything else on her face fits the grid pefectly. Of course if everyone's face met the ratio exactly, we would all look pretty much the same, and in most cases it is what makes someone stand out (and therefore memorable) that makes them interesting. Actually the word "attractive" may be more accurate, although it is definitely not the same as "beautiful". Someone can be attractive while not necessarily being "Golden Ratio" beautiful, and we all know that just because someone is beautiful, it does not necessarily mean they have a nice personality or are interesting, so it seems that other criteria are at play in defining an Ideal Lady...