Beetroot and orange is the pair I reach for when I need to get through a long afternoon. The beetroot gives it that deep red colour and an earthy sweetness, the oranges lift it into something you actually want to drink. Two ingredients, about five minutes, and you do not need an expensive juicer to make it.
Beetroot juice separates as it sits, so stir the glass before you drink. If you want it colder, chill the beetroot and oranges in the fridge first rather than adding ice, which just waters it down. Two medium beetroot is roughly 300g, and you should end up with about 450ml of juice, which is two decent glasses.