So Coinbase is risky to keep btc in then? I have my btc in their wallet.
It doesn't really matter what kind of exchange or wallet service you are using to store your coins in. In all cases you don't own what you actually have as the exchange or wallet service has full control over your coins. Coinbase is well insured against thefts and all that, but even while knowing that, I would never store my coins at any place beside offline where I have full control over every Satoshi that I own.
So what offline wallet do you recommend?
I have only been using Bitcoin Core so far, which is also what I recommend you and all other people to use. I however have to place one important side note - Bitcoin Core requires you to download and store the entire blockchain on your HDD. It may take one to three days to download the entire blockchain, which also depends on your connection with other nodes. If the nodes you are connected with are slow, then the whole download process will severely suffer. In most cases it only takes one day, where some people even manage to do it within 24 hours.