Which bike computer is best purely for navigation? Priorities being:

clear navigation prompts/pop-ups;
screen size;
map layout/colouring/clutter(lack of);
calculation speed.
Your phone would be great for navigation, but not so much for working with sensors. I have had good luck with Garmin (1030 - I figured if I was spending that kind of money, do it once vs. getting a cheaper model that I may want to upgrade later) head units, but I do not primarily use it to navigate. I like the metrics it provides.
I usually plan my route using google maps and memorize it. If I get really lost, sure GPS helps and also I usually have my phone in my saddle bag so I can navigate a route if I need to on the fly.
Regarding speed GPS will always be less accurate than a on bike sensor and because I use a on bike sensor, I do not rely on the GPS speed so cannot speak on its accuracy.
As Chris mentions above, bike computers do involve a lot of personal preferences too.
Also, if you are new to biking, it may be prudent to hold off on expensive head unit purchases until you really feel it is worth it. For me I waited several months before buying my Garmin unit and before that used a cheap $69.00 bike computer for basic speed/cadence. Again, I planned my route on google maps, memorized it, and used my phone for backup if I got lost or made a wrong turn, which occasionally has happened.