Which bitcoin wallet should I use?

There are three general categories of wallet apps: "cloud wallets", "local wallets", and "cold-storage wallets".

Cloud wallets
Cloud wallets store your bitcoins keys on the internet. It is convenient, can't be physically "lost", and can be accessed by multiple devices simultaneously (phone, computer, tablet, whatever). Coinbase has a pretty good bitcoin app that is cloud-based. You can buy bitcoin and ethereum directly from Coinbase, and it's insured from theft and hacking. Another good cloud app I like is the blockchain wallet which also allows bitcoin purchases.

There are three downfalls of using cloud wallets: 1) Your local bitcoin account is easier to "hack" if the perpetrator manages to either guess your password or put a virus on your computer. (Please use 2-factor authentication for an additional layer of safety). 2) cloud wallets are not private! The company that manages the cloud wallet will know who you are paying by analyzing the bitcoin addresses. So if you visit, say, a gambling website, you might get a message like this:

3) Sometimes the bitcoin wallet company gets hacked. MtGox and Binance are two examples of this. If the cloud wallet company is not properly insured or if the severity of the hack is too high, you risk losing your investment. So if the amount of money in your bitcoin wallet is too much to lose, consider getting an offline wallet such as a trezor.

Local wallets
Use this option if privacy is important. Local wallets store your bitcoins physically on the phone itself, which makes it difficult for corporations and even nation-states to spy on their users. It's also more impervious to the aforementioned cloud-based hack attacks. The private keys are kept locally on the device's hard drive, so be sure to properly backup the wallet, otherwise, you can potentially lose your bitcoins! Mycellium and Electrum are good local wallets to use. They're safe, well-known, well-tested and relatively easy to use.

Offline wallets
Offline wallets (aka "cold storage wallets") are good to use if you have a high amount of bitcoin that needs to be kept safe. Two good brands to use are Trezor and Ledger. A trezor looks like this:

The private keys never leave the unit, so your bitcoins are protected even if your computer gets a virus. The only "hack" is for the robber to physically steal the unit. Anyone using cryptocurrencies as a long term investment should get one. The price can range from $45 - $160. Trezors and ledgers can store a variety of crypto-currencies other than bitcoin such as ethereum, dash, zcash, etc.