Lowarance, Garmin or Humminbird?
I don't think that you can go wrong with either of them, if buying new.
As stated below, the key is understanding your graph. Pick one that you find easiest to use and learn it well.
The guy who understands what a 10-15 year old Garmin is telling him will do much better than the guy who doesn't have a clue what a $2000 Lowrance or 'Bird is telling him.
With the linking capabilities that exist in trolling motors Lowrance and Humminbird are going to keep pushing each other which is good. As a result of this linking compatibility you'll probably see more people switching to 'birds because iLink is a little bit more of a known commodity and there simply are more Minn Kotas out there than Motorguides.
Both Lowrance and Humminbird make good units. It depends on what the user desires in a product. Garmin has sort of been left by the wayside a little bit but again...a guy who knows how to read and maximize a 10 year old Garmin will do a heck of a lot better than a guy who doesn't know how to use the most advanced Lowrance or 'Bird.
I had a pair of HDS touch units that I got at a very reasonable price. A guy made an offer on them that I couldn't turn down and I was able to make some money on the sale. Tossed the good old Eagles back on and haven't batted an eye. I'll probably upgrade to 'Birds not because I think they are superior to Lowrance but instead I have a Minn Kota and it would allow me to upgrade to iLink.