I agree with Dwtim on both recommendations. However, the Acute has a small flimsy base that not everyone cares for in a harness. We do not have a problem with it but it probably is a little less stable in a harness. A similar dildo to the Acute but with a larger base is the Silk Medium. The Silk Medium is the same diameter but is perfectly smooth and has less of an angle. Any of those wold be fine choices.
What are your long-term plans? You don't want to get something that you'll rapidly outgrow.
Do get silicone, and start with something smooth. If you're a guy you might want to look for something with a "head" on it for increased prostate stimulation. Get a comfortable to hold, substantial base.