For exceptional strong martial artist who is specialized for unarmed combat the Zoan fruit would be most convienent power up.
There are some nice paramecias who suit for strong fighters well such Luffy's gomu-gomu rubber power. But paramecias are quite unpredictable and random also unless you have 100% identification of fruit you might get something extremely special power such as Brookes 'life' fruit, magnetic fruit or Buggys split fruit, though those fruits have their uses it can be difficult to get full advantage of them in straighforward combat.
Zoans enchant fighters excisting values, for average human who eats leopard fruit he becomes quite strong, but for Lucci who already is superhuman strong eats leopard fruit, he gets his powers multiplied becoming horribly strong.
Enies Lobby gave us good glimpse how benefical can zoans be for martial arts fighters.
Buggys power allow him to get sliced and his body might split for 20 meter radius and thats it. It gives him instantly nice boost of power but it doesnt get more powerful after gaining it initially.
Zoans scale with user, if you get more stronger and more faster then you you will be much more powerful and faster after zoan transformation.
Zoans also seem to have atleast 3 diffrent phases of transformation.
1. Human with some fruit animal properties
2. Human-animal hybrid
3. Animal form with some human properties
But with some drugs and medicines such as Choppers rumble balls you can allow to call even more forms for more special uses.
Zoan is basically enhancing/reinforcment power gives lets say 100% boost for your physical powers.
If your basic strenght is 5 then your zoan transformation strenght is 10.
But if your basic strenght is 200 then you get strength of 400, well 100% might be way off but its basic image of it.
There still remains exceptions, some paramecias are more better for fighter than some zoans but generally zoans are best suited for fighters.
I think if Sanji would HAVE to get some fruit then Zoan would be best for his fighting style (unless he gets bird-power then he is quite f*cked
).