I use a Grado Reference Platinum on a Linn LP12. It's not perfect--probably a little too warm--but there is a sort of magic and involvement that I haven't heard with many other cartridges.
Another Ortofon worth considering would be the 500 series. I'm considering trying on my LP12. These are harder to come by, but they are supposed to be better than the OM series. I think the styli can be upgraded (say, getting a 520 to replace a 510 stylus) like you can with the OM series. Even better, they use closed mounting holes, which can make installation a lot easier.
Another Ortofon worth considering would be the 500 series. I'm considering trying on my LP12. These are harder to come by, but they are supposed to be better than the OM series. I think the styli can be upgraded (say, getting a 520 to replace a 510 stylus) like you can with the OM series. Even better, they use closed mounting holes, which can make installation a lot easier.