I'd want someone who can guide me through the pitfalls inherent in Instrument training. An excellent knowledge of IFR regs and how to apply them is a must. The instructor should feel comfortable operating in the IFR environment. Bottom line: your instructor needs to be able to take you from a basic instrument scan to being able to pull any approach and shoot it to minimums so you can get on the ground safely if the weather goes south on you. If they understand the system and can show you how it works, you'll become a proficient Instrument pilot and be safe.