But you should have several characters because it enriches you to more gameplay variety and gives you more chances in getting specific transmog/titles/etc easier.
As nobody has written anything about it… yes, there are hidden and locked rewards behind each class across the whole game. Unfortunately I don’t have a list at hand but the most obvious ones are the specific items from the Legion Class halls and transmogs for Paladins (like Uthers iconic hammer) and Death Knights.
In the end, you should strive to have at least every class covered, even though you will only play them for a transmog run at best. Most of these characters come extremely in handy during seasonal events and doing the Loremaster, therefore never write them off.
My suggestions would be:
Human Paladin LV110 (iconic race/class combo, you can level the character for the BfA-achievement Two Sides to Every Tale - and humans are the best for unlocking reputation-stuff, like Allied Races)
Either a Nelf/Belf/Dwarf/Troll Hunter, also very iconic images (Survival or Beast Mastery if you’re new to the class)
Additionally a Forsaken/Worgen DK (Frost), also very easy to play if you’re new to them
Create a LV98 DH, even when you will never play it. You’ll lose the opportunity to skip +97 levels coming with Shadowlands. Do this on both sides (nelf/belf).
Don’t waste these opportunities, you paid for the boosts. A normal BfA-licence only costs around 15 Euros while an ordinary boosts is more like 60 Euros.