Just out of curiosity, I would like to know which BTC Wallet is the Most used.
I prefer to use blockchain.info because it's a trusted wallet that has been around for years. It is probably one of the earliest online wallets I've used as well, from memory.
It is pretty simple to use, especially with signing messages, importing and exporting.
You also get access to your own private keys, which is always good. Others like coinbase doesn't offer that option.