YouGov's polling has been interesting. They've tested a full field, head to head, and likely the most realistic scenario: a field of a handful of probable candidates. Here's what the last option looks like. Closer for DeSantis than a full field, but again not enough. One caveat with this polling. It is surveying adults, not registered voters nor likely primary voters. If it is just Trump, DeSantis, and say Haley, again Trump wins, but it is fairly close.
One very large caveat in general with primary polling is that the candidates polling right now in the single digits can certainly improve as the primary gets closer. Some of these candidates have low name recognition. Polling will change as the field becomes clearer. For example at this time in 2015, Cruz and Rubio were polling in single digits. They eventually would poll in the double digits and would secure a not insignificant number of primary votes. Let's not forget Ted Cruz managed to acquire 551 delegates and won 11 contests in 2016.