A portrait is a life-like tattoo rendering that more closely resembles a photograph than a painting or drawing. They are typically used as a memorial tattoo to honor a loved one or family member, which is why they are made to look so realistic. Portrait tattoos are highly detailed and often considerably large, so the outside of your upper thigh would be the most ideal spot on your leg. This way it can lay relatively flat and be in full view without wrapping around the back or front of your leg.
A band tattoo is a design that wraps partially or completely around a portion of your body, such as an arm or leg. For a leg band consider wrapping it around either the top or bottom portion of your calf so it is more visible than the thigh but still easily covered by pants. There are a variety of types to choose from -- tribal, Celtic knots, flames, barbed wire or even script consisting of one word or a full sentence.
If you are a musician or simply enjoy music consider tattooing the musical notes from your favorite song or piece of music. A complete song may be difficult to fit all in one tattoo, so instead determine your favorite portion of the song and have the correlating sheet music tattooed to the outside or front of your thigh. This is a very exact tattoo with a multitude of perfectly straight lines, so you should only employ a very experienced artist for the job.
Vertical script is a style of tattoo that reads top to bottom instead of left to right. The ideal vertical script is only one word since space is somewhat limited and breaks between words can be difficult to see vertically. Choose a script that does not require each letter to flow into one another -- calligraphy and graffiti script are sometimes difficult to read vertically since the letters are typically designed to flow together. Instead think of one word are sums you up and choose a simpler font going down any portion of your leg -- thigh, calf or shin.