at the end of the day, i think the one you can do best is the best graft…
not the one you read something about here, not the one you heard good things about on youtube, the one YOU can make behave. There are exceptions, nut trees have a reputation for being pretty finicky, but apples and pears are not, so whatever you do well is going to be a better graft than whatever you do poorly.
I know you were asking with the hopes of “picking one and improving your skillset” and that’s not unreasonable, but there’s very little universal truth, universally – a piss-poor whip and tongue isn’t going to be better than a meticulous cleft graft, for example.
Now, different grafts have different strengths, including their quickness to “take,” ability to use mismatched scion/rootstock, etc., but yeah, there really isn’t a “best” graft, imho. Also, as a side point: if you can dodge a wrench, you can dodge a ball…just ask Patches O’Houlihan. For folks not familiar with the quote, it doesn’t matter…the point is learning one graft isn’t somehow zero-sum, where you aren’t getting better elsewhere…if you go from poor to pretty good in bark grafting, that increased ability to manage your cuts, cambium alignment, wrapping and aftercare, etc. isn’t somehow “useless” if you need to do a whip, or a cleft.
I think the honest answer is there IS no honest answer to your question…that said if you really want to focus, because “you gotta start somewhere,” if anything I would pick 2 techniques, a “scion matches rootstock” like whip or whip-and-tongue, and a “mismatch” technique, like cleft, bark, or chip that lets you work with grossly mismatched scion/undestock situations… But you’re likely to find any grafting makes you better, and you’re also likely to find you can’t always get that reductive. I do probably something like 70% whip and tongue as it’s my most comfortable, but I also do bark, cleft, “offset whip and tongue”, and chip budding to make up the rest–what I do in any given situation depends on things like the scion and rootstock diameters, # buds, position on the rootstock and “accessibility” (I do a fair amount on in-ground plants), etc… the materials themselves, their position and relative accessibility, my ability to contort myself/move the rootstock, etc. tend to determine the “correct” graft for a given situation.