What's your budget?
Do you want networking with your outboard? (Will also depend on the age of outboard)
Each have their strengths from what I've seen and read, Hummingbird has the best side imaging, Lowrance has mapping, and Garmin has the best traditional sonar.
I would definitely argue Garmin has the best user interface, settings and navigation are extremely easy to follow, I would definitely lean that way for a first unit.
I'm on all their FB users groups, there was some great sales from BPS/Cabela's with Garmin units for black Friday.
They had the Echomap UHD 93USV for 700$
That's 9" touch screen AND actual buttons, sideview, downview, sonar, and mapping. With the option to add livescope one day