Depends on your budget and the features you want .....
Both Lowrance and Humminbird are excellent brands .....
To have mapping - you require a GPS unit. A great mid market unit - with all the bells and whistles you are looking at >$1000. Here is a great option ....
http://ca.humminbird.com/products/77...ci_HD_SI_COMBO
This unit gives you GPS and mapping, high def, down imaging, multi beam sonar, dual card slots, side imaging, sonar etc....... and it can be networked to other units via the use of an ethernet hub and cable.
Humminbird also allows you to control your MinnKota Motor with i-pilot link equipped motors, controls and/or oscillates your cannon down riggers (cannon-link equipped) all without leaving your captain's chair.
Best value for the money right now is this unit ......
There are some good options that are cheaper, but will not provide full features like these in most cases.