[Samsung Galaxy S21 Ultra] enigmaamit:
Most of you make very valid points, and there's some truth to all the different recommendations made above.

However, every person will have a certain work and life setup where one or the other method may not be suitable as compared to others.

A lot has been written about battery degradation with fast charging and it may well be true. Even I subscribe to the philosophy that slow charging is better when possible. However, sometimes you need a quick top-up, and it's ok to boost.

Let me try an analogy: Just like a car gives maximum fuel economy, least pollution and best engine longevity at lower speeds and optimum RPM, sometimes you just want to get somewhere in a hurry and it's OK to floor it within a reasonable limit, or push it to a 100 mph on an interstate highway. Running at 7000 RPM for a quick dash isn't going to burn the engine out, but you can't communte to and from work everyday at that level of engine stress. Most good cars can handle both scenarios.

I've owned an OnePlus 6 prior to this and have only ever charged it with the supplied Dash Charger. For 3 years, the phone charged every single day with the fast charger. Sometimes 2-3 times a day from 20-100% while ripping intense gaming sessions on CODM. It still lasts the entire day on a charge - after 3 years! I'm not saying it is a good idea, but I havent seen it do anything bad to my phone yet.

Decided to be a little bit more careful with the new S21U, so this is what I follow now, based on all your comments and recommedations, and all the reading I've done online about Li battery health and maintenance from sources I trust:

I use Fast charging for quick mid-day boost when required (if you're going to be out and about, away from a charger). I have an 18W power output bank and a 25W charger at my workplace. I use these very rarely, mostly when I have a lot of photos or videos to shoot, as that kills battery pretty fast.

I use Slow charge with cable or wireless charging for regular day to day charging. I've started putting the phone on a 10W wireless charger once I get back from work, before heading off to the shower and then dinner. By the time dinner is done, usually the phone is back up to around 85-90%. Have set Bixby routines to turn off WiFi, Bluetooth, Mobile data, AOD and turn on power-saving mode at night when I'm asleep, and then restore all back up when the morning alarm rings. I hardly lose 1-2% overnight with this set-up, as nothing is consuming battery. So usually by morning I am ready to go with around 80% battery. This usually lasts me an entire workday till I get back home. Rinse and repeat. If i seemd to be dropping below 30% at work, I have my fast charger at my workplace, and a 30 min top-up is usually enough to push it past 60%.

Now this setup suits my lifestyle and may not sit well with the next person with a different routine or a faster pace of life. You may not want to keep your phone for 3 years - if you're someone who changes phones yearly, all this deliberation and discussion is pointless. At the end of the day, you just have to figure out what works best for you, but either way, I don't think the amout of gain or loss in battery health is going to be a huge margin, unless you're abusing the battery by hammering it with deep charge and discharge cycles at extremes of temperature.

Most of the current batteries are pretty solid, and will easily get you 2 years of reliable performance without stressing over every granular aspect of charge and discharge.

But hey, this is XDA. Regular folks don't spend hours on a forum discussing battery electro-chemistry :P. We're enthusiasts, hanging around here to over-think the hell out of every trivial detail there is about phones. So think away, ladies and gentlemen! And let the brain juices flow. It's always refreshing to engage with passionate people.