I'm a big fan of the Ultradot for target-style red dots. They're sturdy and reliable, and are very popular with the bullseye crowd. The 25 mm is pretty compact. It come with rings, but you can put it in lower ones if you want.
http://www.ultradotusa.com/ultradot_dist_blue_006.htm
http://www.ultradotwest.com/ultra_dot_west_2007_002.htm
http://www.bullseyepistol.com/dotsight.htm