In my opinion, mostly coins that had dipped big enough to at least 20% after the dip. If your you purpose to gain big enough then this would be a good strategy. Of course, you still need to evaluate the coin before buying.
Altcoin movements tend to be much bigger than bitcoin moves - so a 20% dip is nothing, they can and do fall 50%, 80%.
Before you make any buys, look at the charts and look at previous lows and previous movements. Sometimes there is a support after it dips 50%, but rarely at 20%. If that is the case for the coin you are looking at, WAIT till it gets to your desired buy price.