Not wishing to get into a debate as to which brand is best but I have owned Lowrance ( eleven different models), Eagle, Garmin and Magellan . I have not used a Hummingbird. All were equal as far as accuracy is concerned. I have found that often the accuracy of a GPS, when navigating to a waypoint, is directly proportional to the experience of the user. The least accurate way to go to a waypoint is by using the map screen.
As for internal antenna versus external there is no difference concerning accuracy. All hand held units have internal antennas and are very accurate. Garmin has always had internal antennas for marine use and they work fine. Installing one with an internal antenna is much easier. The only disadvantage to using a Lowrance/Eagle unit with an internal antenna is you can’t network multiple units if they have internal antennas.
What is the best GPS? I don’t know, but I do know that the vast majority of professional fishermen use Lowrance. I have found them to be more user friendly than any other I have used. I don’t think you will regret buying one whether it is a Hummingbird, Garmin, Magellan or Lowrance. Keep in mind that to get the most out of your unit you will have to purchase mapping software to go with it. You can get the Fishing HotSpots Elite Pro(for Lowrance/Eagle) from Provantage for around $56. Most mapping software will run you a minimum of $100.
Good luck with the brand you choose.