a very common mistake among people who invest in cryptocurrencies and are generally new to investment is that they think "diversify" means "buy multiple things instead of one". but this is wrong and a very simple way of thinking about it.
just buying more than one coin is not called diversification. if it was that easy then everyone would have become millionaires. diversification means you buy multiple assets that are not connected to each other! for example if you have coin A and buy coin B to diversify but if coin A goes down then coin B follows it down then that doesn't mean you diversified. it only means you made things harder for yourself to handle.
Which coins would you advise to combine to have proper diversification?
exactly what @jseverson said apart from the stablecoin part because they are still altcoins and they also have risks similar to altcoins. for instance these days you can see that Tether that everyone mistakenly thought will always be stable has fallen down below $1.
but basically when you diversify, you are diversifying your money that you invest. this means buying different assets that are not connected to each other so that they are not taking influence from similar incidents. because that is one of the points of diversification, to reduce the risk of losing money when some incident happens. for instance when Bancor was hacked this year price of every cryptocurrency dropped (bitcoin dropped less and altcoins dropped more). so if you held cryptocurrencies only that meant you lost money but if you held bitcoin + stocks for instance you only lost money on your bitcoin investment while your stocks were moving the same way as before because they weren't affected by this incident obviously!