This is mostly for bitcoin users because QT (especially of core) got so massive if you get single block file corrupted its taking ages to sync and check balances/send money.

Currently on a very good internet its 3rd day im syncing Windows-QT from zero and i got irritated so decided to move all wallet addresses to Electrum (a wallet that doesnt download all blocks and syncs really fast).

It seems most guides online that show on top of Google are outdated (Electrum now works a bit different) - so lets have one thats best and fastest right now.

1. Open Windows QT

Well QT doesnt have to be windows, can be OSX or Linux but you need to actually open QT and let it start syncing.
This is just to let it load your wallet.dat with all wallets and their privkeys.

2. Get private keys for them

You need to do: Help -> Debug -> Open Console
If your wallet in encrypted put: walletpassphrase YOURPASSWORD 600

3. Start getting private keys

Simple take addresses that you have used previously (they are in history) and believe have money. Then do the command:
dumpprivkey YOURADDRESS

Write down what it said.

4. Import to Electrum

Open Electrum let it sync and go to Wallet -> Priv Keys -> Import
Put there line by line all private key you got in previous step.

5. Check your money

Now you can use this addresses on Electrum and get balances. Fast and easy. Way faster than 3-4 days syncing with few wallet crashes of Core.