Most characters will be more powerful at 20 by being 100% single class.
That said, there are a lot of levels before level 20, and multiclassing can make those go by much easier, and lets be honest, what percentage of characters that start at level 1 make it to level 20?
Lets look at a pretty strong option for multiclassing. Level 1 Knowledge Cleric, level 19 Wizard. You LOSE: 2 3rd level spells that recharge on short rest at level 20. Access to higher level spells and feats are delayed a level (altho spell slots per day is the same).
What do you gain? Expertise in two knowledge skills, light,medium,shields simple weapons proficiency, Access to guidance, spare the dying, cure wounds and healing word, trade INT save for CHA save bonus. (Theres simply not many INT saves, CHA is not as good as wis/con/dex, but much more common than INT).
So even if your character does make it to level 20. Which is a BIG if. How long will they adventure at level 20? Will those 2 level 3 spells per day really be more important than the fact you were able to wear actual armor your entire adventuring career?
Really the only time I would worry about capstone abilities is if we were starting a new campaign at 20.
Who would be must have in that case:
Barbarians : +8 Ability bonus and unlimited rages, those are keepers
Cleric: Maybe.. its cooldown limitation is really limiting
Druid: Unlimited wildshapes means you basically have unlimited HP if you can't get killed in a turn
Fighter: 4x attacks are pretty impressive, especially with the additional action surge
Most of the other ones are methods of recovering resources outside of long rests, which are handy in certain situations, but not really game changers unless your campaign has extremely tight time schedule or some other reason long rests wouldn't be viable (which is more difficult when players have access to high level magic)