Question: sRGB or Adobe RGB 1998?
Why? Because everybody else assumes you're using it.
At least some compact camera's (like my Canon G9) and all D-SLR's offer you the choice between shooting your jpegs in sRGB colorspace or Adobe RGB 1998 colorspace. If you shoot RAW you can postpone the choice between the two until the moment you convert your RAW files to tiffs or jpegs. If you shoot jpegs a chip in your camera translates the raw data coming from the camera sensor into an image consisting of, say, 12 million pixels. Each pixel has three values "attached" to it: one for red, one for blue and one for green. Together these values determine the color of that pixel and all pixels together determine what the image looks like. Your computer monitor cannot display all these colors and your printer probably cannot print them all. What they can render or print differs from screen to screen and from printer to printer. That's why some printers have so many different cartridges: to enable them to print as many colors (as wide a colorspace) as possible.
The sRGB colorspace consists (roughly) of all the colors an average computer monitor can display. Compared to adobe RGB 1998 it is a relatively small colorspace. This does not mean that it is better to use Adobe RGB. That depends on what you will use the images for. For sharing them on the internet, e-mailing them to friends and printing them on an ordinary inkjet printer you must use sRGB. Use anything else and you are very likely to be disappointed. Other people expect you to use sRGB. Actually it's their computers and the software they use, like internet browsers and e-mail programs, that expect you to use sRGB. Those people themselves often don't have a clue what a colorspace is. So don't bother them with images in Adobe RGB colorspace. Move your mouse over the image below to see what happens if you accidentally upload an image edited and saved in Adobe RGB 1998 to a website: dull colors! Always convert it to sRGB colorspace first. If you upload your images to an online photo printing service to have them printed for you, you should also use sRGB. They don't expect you to use anything else and you will very likely receive pale images with dull colors if you do. It's the same for printing your own images on an inkjet printer. It is highly unlikely that a sub 100 dollar inkjet printer with only four cartridges can print all the colors in the Adobe RGB colorspace. Edit and save your images in sRGB and you are much more likely to get on paper what you see on the screen. I own a Canon Pro 9000 printer with eight cartridges and I print from sRGB files. Know why: I simply don't see the difference with prints made from Adobe RGB files.
Image uploaded in sRGB. Move your mouse over the image to see what
it would have looked like had I uploaded it in Adobe RGB.
The bottomline: Use sRGB for your standard workflow. Only use Adobe RGB 1998 if you know exactly what you are doing, or you are asking for trouble (wasting ink and money on bad prints; other people complaining about the colors of your images). If editors of magazines or professional printing labs ask you for files in Adobe RGB 1998 colorspace, then you are probably a professional photographer and you don't need my advice. But just in case someone ever asks you for an image file in Adobe RGB colorspace: shoot raw, transfer the images to a computer with a calibrated wide gamut screen, do all color corrections in raw if the software you are using for conversion allows for it. Then convert the image to a 16-bit tiff in Adobe RGB 1998 colorspace, edit it in photoshop (or whatever you use) and save the final image as an 8-bit tiff in adobe RGB 1998 colorspace. And don't blame me for it if the result is disappointing. I told you to use sRGB!