Bushnell TRS-25 for under $100 if you're on a budget. Aimpoint PRO or T1/H1 for $350-$500 for a high quality one that can serve double duty on a HD weapon (don't need to be shut off and can go years on 1 battery). I have several TRS-25s and T1s and all have been used on shotguns and rifles.
If you are using a light, the TRS-25 will work fine. If you're not allowed to have lights, you may have to go to the T1 route as its adjustments go dimmer to allow for NVG usage. Some of the cheaper RDS produce too much glare even on their lowest setting.
There was a T1 on the Classifieds here yesterday for a great price. Thread #2803563