It should have as many useful features as a good desktop wallet. Being non-custodial is a must. There are good open-source mobile wallets, meaning there is no need to settle for something close-source. The wallet should give you the freedom to select your own fees even after the decimals. Some wallets come with priority fee selections like low, medium, high, and that's far from optimum. Coin control is another noteworthy feature, as is the possibility to freeze certain inputs if you want to. I prefer wallets that have been around for many years and have clean track records instead of chasing beautiful looking designs and UIs, but with questionable qualities. 

Your mobile wallet will depend on what coins you intend to hold in it. I see no point in using a multi-coin wallet to store bitcoin, for example, when we have standalone bitcoin wallets. And don't use an exchange as a wallet because it isn't.