I'm sure everyone here will suggest Bitcoin and Ethereum for the first couple, because we know that almost everyone uses Bitcoin and Ethereum. And as an advanced option you can choose several coins with a high level of popularity and one more, don't forget for the USDT pair.
Not necessarily Bitcoin and Eth. These are good coins and profitable but we do have other options in the form of Litecoin and NEO which I think are also profitable and imparts diversity to the portfolio one holds.