That's like asking someone what car you should buy.
Give us some info.
Most importantly, what do you want to spend?
What will be the primary use of it? Is it really just a depth finder, or are you looking for tournament quality? How and where will it be mounted? Do you want GPS capability?
There are so many options out there, from the most basic $100 units to the $2500 units with 10" color screens and GPS. I certainly won't claim to be an expert on the subject, but with some more info we can help you out.
You may want to look into used units as well, a lot of folks take the electronics off their boats when they sell them or upgrade their electronics regularly, there are some great deals to be had, especially south of the border.
Waxy