Asklemmy

Syncthing for me. It is not cloud though - but better.

Depends on what you want to use it for. As others have said Nextcloud is super popular, I'm also a fan of https://syncthing.net/ to keep stuff synced across devices

I run a Nextcloud instance on my Unraid server.

I've been using mega synced folders for most stuff. Works fine.

NextCloud running in docker on my server. I can then sync folders from both my desktop and phone.

I've have good experience with MEGA, they were offering 50GB free when I signed up, I think they're offering 20GB on their free tier now. Your files are stored on disk locally while also avaliable on the cloud via their website and mobile apps.

Hetzner Storage Box.

I've been using Filen, seems to work pretty well, it's got a Linux version of the desktop sync client (comes as an AppImage IIRC) and I dunno if they're still doing it but they used to have a good price on lifetime plans that were ~100GB that you could stack, so I got a good amount of storage without having to pay a monthly fee.

Nextcloud with a group of privacy advocates bros. 25GB

One trick that's not google is use a Android TV box with a mouse and keyboard and a single app by Banana Studios called FTP server buy the pro version $1-2 app on Google Play Store and your golden you set the ftp account admin then other accounts on it for people you want to have access to certain folders you need root on your TV box and then you can set it to auto port forward aka port trigger port 21 (press a single button in the settings) turns on the router port without doing it manually, you can use it on your home network aka your local network but it also provides an automatically generated outbound address to use for connecting to the ftp over public internet you do also have unlimited user accounts you can create with passwords for them that you set so it is ok on security not ideal but ok.

You can easily grant specific access to individual folders in a specific place you choose on the devices storage, plus you can adopt a hard drive in Android to store it all in like a 1tb or bigger drive (even ssd's) over USB3 then a second drive to have it on occassion plugged in and you'd copy and paste the folders to this spare drive incase the android hdd dies ever because you cannot restore android TV really using apps like nandroid in TWRP on phones it can't seem to be done some people may be able to on certain TV box devices not many have managed it though yet, but you can re copy the data to folders manually on a plugged in usb drive and have another permenantly conmected as the adopted storage aka main storage/internal stirage of the TV box so it can be huge internal storage for apps and data. And then if the drive ever dies for the adopted storage you add a new drive via usb and re setup the OS and the ftp server app run the ftp server again and copy and paste out the data from the spare hard drive using a file manger you just generate the folders in the file manager and then copy and paste the data into them the spare drive to the new adopted storage internal drive and your done.

But for your self it's easier than making multiple accounts and some of these sysncthing style tools do allow you to use your own ftp server to store on them as well so if you have one of those you'd make a folder to backup up on one of those and connect it to the ftp it will work on your local home network using the none public ftp address even when the internet is dead but the router is still switched on and working as long as it has a network to join in the app be it with the ISP on or not it runs, you could just use some WIFI to WIFI app then for sending files to the TV box when the router is disconnected from it's phone line and or the router is powered off it wouldn't matter for offline use phone to phone android TV box to android phone etc then and or online use either good apps for file sharing are localsend app or nitroshare app on F-Droid for android TV and phones to transmit files they just need to join the router but internet is not required just the WIFI through the router being on and both devices are joined to that same router aka local network.

This is my cheap alternative to a proper ftp device or nas drice really but is really cheap practically zero cost to leave it on if it's an Androdid TV box they only uses say 7.-15 watts averaging about 10 watts of electric consumption.

Other options might be a server like nextcloud on a Raspberry pi or on a PC or laptop but PC's and laptops drain electric and they all burn out usually eventually costing a lot more to replace than to replace a TV box which cheap reasonable quad cores are around $40 they need no batteries so doesn"t breakdown from battery swell like a phone will in a year or so and use maybe 1/6 to 1/10 of the electric of a laptop and multiple times less than many PCs

Proton Drive is working well for me so far.

I use iCloud and Sia.