Vague wording aside (tf does "expressed himself best" mean?), this is actually a fun debate to be had here. Djokovic has been exceptional at the Masters, so you really have a good selection to choose from.
In his prime, Miami was the clear standout. Some of his best wins against the toughest opponents came here, particularly the 2011 final which is one of the very best Bo3 matches out there (and a much better match than the IW final just a few weeks prior). I suppose the first couple of sets in the 2015 final weren't too bad either, even though it was Murrovic. Plenty of wins with few or even no sets dropped, like 2012, 2014, and 2016. Personally, Miami has probably my least favorite playing conditions out of any big tournament, so I don't find any of his top performances particularly exciting to watch (except the aforementioned 2011 final). But you can't deny how impressive he was here. Even metsman has to begrudgingly take Djokovic over Fed in daytime Miami conditions!
His one problem at Miami: he pretty much stopped taking it seriously from 2019-on, same with Indian Wells. I think he'd still do okay if he gave the tournament a solid effort like he did with Rome, Cincy, and Paris in his post-prime, but he just chooses not to. It's hard for me to rank it as his best with that going on, though I'm still tempted to.
You can also make a decent argument for Paris. Djokovic put in a very good series of wins from 2013-2015 (Djokovic in general was an indoor beast in 2012-2015, besides the super dumb loss in Paris 2012), and there were some fun victories stacked in there like the 2013 SF-F combo. Paris has also been the host of some of Djokovic's best post-prime performances, like the 2021 Paris final against Medvedev (the pair's only legitimately great match, IMO). Even in 2018 where he was somewhat ill, he still had a memorable road to the final.
The problem is that his competition in Paris has been pretty consistently awful. Good draws just don't happen in the last Masters of the year. I think the 2009 and 2021 finals were perfectly fine, and the 2013 tournament across the board wasn't too bad, but Djokovic's other wins have mostly consisted of him doing just what he needs to win and not much beyond that, against hapless opponents like Shapovalov, Dimitrov, and Raonic who can't really do much damage to Djokovic if he's playing above first gear.
In contrast, Djokovic has faced some of his strongest Bo3 competition at Indian Wells, and he's still amassed quite a few titles there. I rate him just a little bit behind Federer there. I think there's a lot to like about every one of his wins at IW. He took down some solid competition in 2011, 2014 (I'm a fan of his final with Fed here), and 2015, and his 2008 and 2016 wins, while not achieved against the greatest of competition, were still commanding enough to deserve respect. Even so, he was a little shakier in his title runs than in Miami. Whereas Djokovic saw clean, decisive victories in Miami, he had to struggle a little bit more to get to some of his IW championships. Some of those struggles make sense: dropping a set each to solid versions of Federer and Nadal in IW 2011 doesn't exactly raise eyebrows. But dropping four sets in 2014 does. So strictly compared with his Miami success, I think IW can be ruled out. And his IW bid suffers from a similar problem as his Miami bid: he just gave up after 2018.
Shanghai is kind of a sleeper pick. The tournament hasn't been held for too long, and it was out of action for three years during COVID which took away even more opportunities. But Djokovic has been pretty solid here. The 2012 and 2013 finals were both great performances against strong opponents, and 2015 and 2018 were nice clean wins too. I don't think Novak's serve was even broken in the 2018 edition. And there weren't too many terrible losses either. Between 2010 and 2016 (exclusive) the only one he had was to Fed playing his best baseline match in years. Had he had the chance to play it more often, I think you'd see a few more shoutouts in this thread. Djokovic's Shanghai performances are great demonstrations of his prowess on a quicker court. But while there's a natural degree of speculation in this debate, I feel like you have to lean into it too much to pick Shanghai here, so I won't.
Taking everything into consideration, I think you ultimately have to give it to Rome. Djokovic has been very consistently good here, and this extends across the entire length of his career, not just his prime.
You have the 2008 win and the 2009 final, some of his first major clay court accomplishments. While they weren't the most impressive of performances, they do demonstrate an early aptitude for the tournament (like with his early success at IW and Miami).
Then you have the 2011 win which is IMO Djokovic's very best Masters victory. He won an excellent match against Murray (quite the surprise) in the SF, and he destroyed a not-bad Nadal in the final, his absolute finest clay court performance. And in 2012, he played a great match against Fed (though Fed was poor) and gave Nadal a tough fight in the final.
Then you have 2014 which was another excellent thrashing of a not-bad (though somewhat unimpressive) Nadal. This one was more competitive.
I'll admit, 2013 and 2015 were kinda mid showings. But they still add to the consistency argument.
But then you have 2016 where Novak fought off a well-playing Nadal who had started to recover from his trough in 2015. Even if Djokovic ultimately lost in tame fashion to Murray, this win was pretty nice.
And even in 2017, Djokovic's awful injury-ridden season, he still destroyed Thiem and Delpo (probably his best matches of the season) on the way to the final here. In the next year, he made it to the SF where he went down to Nadal in a reasonably competitive match.
Djokovic got kind of screwed with the scheduling in 2019, but he still put in a number of impressive matches, especially the Delpo one. And even when tired, he won a good, hard-fought set from Nadal in the final.
Looked kinda impressive in 2020 and 2022 as well despite the poor opposition. Not too far from the 2015 final.
And I'd also like to direct your attention to 2021. That final with Nadal was lowkey a significantly better match than their famous RG encounter not long after. Djokovic did a great job keeping up with the old clay legend from start to finish, even scoring a nice breadstick.
He's fallen off a bit, as his 2023 and 2024 results might show, but his resume is already super impressive here. For the holy trinity of consistency, longevity, and peak level, I think you have to go with Rome. While Djokovic might have put up better overall results at Miami in his prime, I think the sheer volume of success he's had at Rome across his whole career (while, again, still being fantastic at this tournament in his prime) leads me to the previous conclusion.
