Disclaimer: If you don't care about min/maxing and to you the game is about the story and the character second, this thread doesn't apply to you. However if you like to play mechanically strong characters (and also enjoy story), then read on.
My DM has been creating a lot of custom monsters recently and I have been testing out various different classes against these monsters. While these are not true adventuring day style encounters, we also put a bit more into these encounters than simply using a straight up arena style fight. We use maps, minions, and have "goals" for each little encounter, sometimes with secondary objectives, which always seems to be saving civilians with my DM.
I've always known that Party Optimisation in 5e is strong, but after using various combinations of classes now together, it's apparent to me it's *much* stronger than individual optimisation.
For example, running the following party - Lore Bard, Veng Paladin, Abjurer Wizard, Light Cleric - consistently overcame most challenges *easier* than many other combinations I have ran, even DPR heavy ones (Such as Crossbow Expert Fighter + SorcLock). And in general, the more spell casters you have in your party generally the easier higher level combats are.
I found parties that focused more on martial abilities could do amazing damage on paper, but were constantly "fouled" up in encounters and unable to make the most of their damage potential. In one battle I created a Frenzy Barbarian that could do amazing damage, but he was swallowed and his damage potential was neutered. Not only that, due to lack of spell casters, I couldn't really do anything about the situation except pile on the damage and hope I rolled well. Not a good strategy.
Some Thoughts:
I really really now appreciate Lore Bards cutting words. This ability is so strong at higher levels when things can do some seriously nasty stuff to you that being not hit is far superior than soaking the damage.
I really appreciate the Paladin aura even more than I did before, and in fact, I rate the Paladin as the strongest overall martial class you can have in your group, more so than a crossbow expert Fighter, due to the "Group Help" factor he brings. Vengeance Paladins being able to Misty Step as well makes them extremely good at getting out of tricky situations (such as being swallowed or grappled).
I tried running a few different Cleric 9/Other Class X builds, and I really appreciate a full level Cleric much more than I did before. Yes they don't do great damage with their weapons, but not having access to Freedom of Movement, Greater/Lesser Restoration, Dispel Evil and Good, Heal, and a whole host of other useful spells can make higher level combats way trickier than they need to be. Clerics are far more useful than just bless bots.
I find the Wizard far superior to the Sorcerer later on (and even the SorcLock). Yes, you can do some amazing damage with the Sorcerer - I was pushing 120-150DPR going 'Nova', but I quite often sat there looking at my spell list when my party was in trouble thinking "Hmm, not much I can do here". However if being a blaster is your play style choice then this class is for you, I won't take that away from you. I did find though the ability to 'twin' buffs is nice, but I found with the action economy it's usually just better to kill stuff quickly.
It's definitely not all about damage, not as much as I used to think it was. Yes, damage is still the main way you win an encounter, but you also need to think about recon, mitigation and survivability, because the odds aren't always in your favour.
Don't trust that a lot of the optimisation builds aimed at character optimisation will give you an easier time. That amazing DPR won't help you when you're feared, swallowed, held, or knocked unconscious and there's nothing your party can do to help.
Now this is aimed at the "combat" pillar of the game obviously, but I do know the Bard/Paladin/Wizard/Cleric combination also has great utility out of combat in the campaign I run, be it social or exploration, and party optimisation is just as important for those pillars as it is for combat.