X-Rite are the premier brand in colour management (and with the acquisition of Pantone, perhaps the leading brand in colour specification as well!).
We've been selling X-Rite equipment for almost 15 years, and their devices have always proven to be better than the competition. They have the industry standard product in most areas, and after-market software always supports X-Rite devices so you can't really go wrong even if you decide you want to move from a driver printing system to a RIP - I don't think we've ever encountered a RIP that doesn't support X-Rite measurement tools.
The i1Display Pro is the industry standard professional monitor calibrator and we think the obvious choice for almost any image maker. It's fast, accurate, easy to use and very reliable. Compatible with everything.
If you need a more affordable option, X-Rite offer has a more budget options. These are quire a bit more limited, and if you ever see yourself moving to a monitor with direct hardware calibration, they're not a good choice as you can only use them with their own software, so it may be a false economy in the long run. In general, we very strongly recommend the i1Display Pro as a much more future proof option.