It depends on what you mean. The SH he likes the most to draw, the one he likes the most as a charcater etc.7
Probably if it orignated here it should be about panel time and powerlevels. Anyway it is a sage bet to say that generally it is Luffy the SH Oda likes the most.
At the end of the pre time skip he had whole arcs with Luffy alone, in the post time skip he had no problem to forget for years about Sanji and his group first and Zoro and his group later. And even not considering that Luffy is by far the one that in an arc (every arc9 get most panle time, is the one to shine the most (just look at the year of Sanji). Powerlevel wise he is the storngest, fight the main villain (he is the MC) but also is the one who gets a power up each arc (G4, FS, Adv CoC) while even the other 2 most prominent fighters (Zoro and Sanji) barely get any fight, let alone a power up (even at this point what new stuff has Zoro shown? ISDS maybe? Yet the strongest move he has seems still to be Asura, more than a decade later since it was first introduced)(let's not even talk about Sanji).