For me it would have a rich thick mouthfeel. It would be buttery.
It would have a decided sweetness right upon entry. The sweetness comes from using the best white oak available and in knowing which barrels can offer that sweetness. Therein lies the vanilla, the maple syrup, the brown sugar, the caramel, and some of the dark fruitiness that is so intriguing and wonderful in bourbon.
The sweetness is part and parcel of the thick mouthfeel............it is the viscousity that gives the sweetness the staying power.
The bestest bourbon will have a 'spiciness' that will begin about mid palate and will distinguish bourbon from every other spirit in the world. I suspect that people who are not familiar with bourbon mistake that spiciness for 'rawness' and a lack of 'delicacy' and 'subtleness'.
NOT SO! The 'spiciness', however it is achieved (probably most easily through rye in the mash recipe), is essential to bourbon and is a keystone to bourbon's American character.
The bestest bourbon cannot have an abbreviated finish. In my humble opinion this is why Maker's Mark falls short of the mark. It is not a complete bourbon. It is my amateurish opinion that wheat bourbons are at risk of falling off in the finish. That some succeed spectacularly (Pappy 15 YO) is a testament to the skills of their creators (I have no interest in who made the bourbon here..........it is assumed that the master distiller knew how to make good wheat bourbon............it is in the 'decider' of which bourbon is way better than average that I find the mastery). I believe that rye bourbons 'hold' toward the finish, on average, better than wheat bourbons.
The bestest bourbons must have 'balance'. This is difficult to quantify. It is easy to say that the balance lies twixt the sweet and the spice, but that is not all there is to it. Balance is where they work to a common goal..........one might state it negatively...........nothing is allowed to overpower any other taste in the mix. The sweetness is not cloying, the spice is not overpowering, they are not at cross purposes. It is not one agin t'other, it is both together as in a harmonic convergence. Balance is absolutely necessary for a 'bestest' bourbon.........maybe above all else.
The bestest bourbon will have some amount of the char from the barrel. It need not be a lot (too much can spoil the broth.....e.g. Jack Daniel whiskies), but there should be at least a shadow of the burnt wood or char revealing itself. That it is barely detectable is no problem. If it hides from you, look deeper, it can take some effort at times, to find it.
I submit, some may disagree, that there must needs be some of the oak alive and thriving in the bestest bourbon. How much is dependent on individual tastes. I do not mind a significant amount of oak playing about with the other flavor components, others object to that much. It is here again where the Van Winkle bourbons so often excell. They are aged more than most bourbons and being in the wood longer, they become more familiar with it and through the expertise of the family legacy, extract its best qualities.
The bestest bourbon can and will take advantage of the flavor opportunites offered by the yeast in the mash process. Yeast in bourbon is probably the most subtle, and therefore most difficult, to assess as it affects the taste of bourbon. In my limited experience (augmented by my experience in brewing and drinking beer) yeast can add a softer and fruity quality to the flavor of bourbon. It seems to moderate the spicy and fruity elements of rye more toward the fruity side of the spectrum. It adds no bite to the taste, but almost adds a thickening softness much as if bread were added.
I offer for your consideration three bourbons that achieve the bestest status. You will have you own candidates for the bestest, and mine are not the only bourbons that I 'love' but are good representatives of my criteria for what makes for a 'bestest' bourbon.
Unfortunalely, several of the bourbons I put forth are either no longer available or have limited distribution and I lack suitable alternatives for some of them. Maybe some of the knowledgeable BE folks can offer alternatives or their own suggestions.
For a rye bourbon you can do no better than Wild Turkey 12 YO (I have been a bigot for this bourbon for a long time). Since it is no longer available try WT Kentucky Spirit in its place. WTKS is most as good.
For a wheat bourbon you can do no better than Pappy Van Winkle 15 YO (which is in short supply). If you cannot find PVW 15 YO, try Van Winkle Family Reserve 12 YO Lot B (which is also in short supply).........others from BE may offer some alternatives to the two Van Winkles I have suggested as first rate wheat bourbons.
For a bourbon where the yeast plays (along with the rye) a significant role, I would suggest trying the Four Roses Small Batch or the Four Roses Single Batch bourbons.
Unfortunately, I think the Four Roses bourbons are only available in the U.S. in Kentucky. I take, and find, the opportunity to get these bourbons. They are very much worth whatever effort is needed to get them. If bourbon is important enough to you, you will fine a way to get some to satisfy this itch for a bourbon where the yeast makes a significant difference in the flavor.
I am not a conny sewer of bourbon. But I have tried many, many bourbons and have some knowledge of current offerings. I am quite confident that what I have recommended to you in the way of 'bestest' bourbons fits the bill.