In terms of sheer damage numbers, like the other answers are saying, Melt or Vaporize are the most effective. Melt is cryo and pyo, Vaporize is hydro and pyro.
To get the maximum damage boost, the 2x multiplier, for either of those reactions, you should apply pyro last. You can get even more damage off those reactions if you have high Elemental Mastery on your pyro character or artifact set bonuses that boost reactions, like the 4pc Crimson Witch of Flames set.
Of the two, Vaporize is more reliable because hydro is generally easier and faster to appy than cryo.
The current meta of Genshin heavily favors high damage output because of timed domains like Spiral Abyss, where the amount of rewards you earn directly correlate to the amount of time you have remaining after killing everything. Reactions with low or no damage have never really been in favor.
For example, Crystallize (geo and either pyro, hydro, cryo, or electro) provides no damage, only a shield if you pick up a crystal that results. Electrocharged, hydro and electro, has a very low multiplier. Frozen (cryo and hydro) does no damage by itself, but breaking the ice with a claymore user, or Klee and Ningguang’s attacks, does.
One character who is commonly considered to be held back by her element is Keqing. Electro, again, had quite low multipliers for any reaction involving it, and physical damage dealers are usually worse than elemental DPSes.
Razor and Ningguang are exceptions to this rule. Razor, an electro character who deals primarily physical damage, has access to Superconduct, cryo and electro, which deals AoE cryo damage and lowers physical resistance by 40% – quite a lot! Ningguang gets around geo’s lack of offensive elememtal reactions by having high multipliers on her attacks and constellations that increase her damage even more.