Which two roles are the most different and why?

Mid lane tents to have the hardest mechanics, due to the amount of burst and it being the shortest lane.

Jungle has the hardest choice making and is easiest to be completepy forced out of the game if you fall behind.

(3rd role but ADC is the hardest to flat out carry with in solo Q since teams wont be able to protect you that well)

Jungle and any other role tbh. If you are playing mid, you can easily pick up top or adc per say or vice versa. Jungle tends to be more unique of a role and playstyle

Top and ADC are probably extreme opposites of one another.

Top has diversity, ADC does not. Top is mostly, if not always, alone against their opponent, ADC is not. Top usually only has to contend with 1 or 2 threats alone, ADC usually has to contend with 2 or more threats with help. Top and Bot are on exact opposite ends of the map.

Top & Support in my opinion, some of the champions might be similar. In terms of mechanics & macro during the early to midgame it's a total different beast.

Top lane and Bot lane. Top lane doesnt matter. Better bot lane generally wins. The game revolves around bot lane. Top lane plays their own 1v1 minigame.