I would echo what @Keepnkeepingon described above, very accurate. There are way too many variables of all kinds that account for what 'best sex ever' means to people. It is not as simple as being in love, or not as simple as this specific person or that one. For example, I know for a fact (as we openly talk) that I am my wife's 'best sex ever', not being full of myself here, she only ever had one partner before me back in university years who was not mature enough to care about the woman's pleasure. But, as a contrast, if I had to make the argument, I am pretty sure I am also her 'worst sex ever', too. How does that add up? circumstances. I remember an occasion about a decade ago, where we were in a rush, I just took a shower, and brushed my teeth with a very strong, minty toothpaste that I did not rinse out of my mouth too well, and as I went down on her, it started stinging her, causing her a burning sensation and discomfort. Contrasting that, I know what she loves and generally make her have a longer, more intense orgasm through penetration and then a second one with my tongue, and she absolutely loves the combination. See? worst and best, and yet, it's the same person. And that's just my own personal experience through the lens of the question.
Chemistry is definitely a big connective tissue here, but so many other things go in the mix, as well...
To be frank, improving abilities I think has not too much to with it all. And while I think "best sex ever" is just very vague, exploring different things together what you could enjoy is a fun thing to do... potentially finding something that makes it an even better experience for you, or both of you..