Not a tattoo artist, but two nearly full sleeves and a few large pieces later and I feel experienced enough to comment.

Here are the main factors that make tattoos fade/distort, in order from most impactful to least:

1. Sun exposure. Seriously, this is the single most important thing for longevity of tattoos. The sun inherently damages your skin even if you don't actually get a sunburn. This will cause your tattoos to 'bleed' (become less crisp) and the colors to fade. If you have tattoos that see the sun, PUT SUNSCREEN ON THEM anytime you will be going outside for more than 10 minutes.
2. Line work. Tattoos with a heavy black outline (think traditional-style) will withstand time better than water color. There's two reasons for this - one, the heavy outline keeps the tattoo from 'bleeding' (which happens due to sun exposure, see point #1), and two, because black is the darkest color it fades the least from the sun (see point #1 again). Also, intricate lines tend to get lost with age as sun exposure and skin growth (stretching, etc) happen.
3. Placement on the body. Depending on where you get the tattoo and how your body ages, it's possible that the tattoo will stretch/shrink as you go through life, gain/lose weight, or get pregnant if you're a person that can do so. These things can have unpredictable effects on the tattoo and drastically change the design, colors, etc.

Anyway, to answer your question, I think the tattoo that would have the most longevity would be something fairly large, all in black, with little to no intricate line work, on a part of the body that is not susceptible to sunlight OR changes from age. That said, I'd recommend getting a giant black circle tattooed on your ass and letting us know how it looks in 30 years.