Firefox most of the time. However, it has two issues:
(1) Full-screen videos usually result in my MBA fans coming on full
(2) It does sync to FF on my iPhone, but nowhere near as seamless as Safari. FF on the phone isn't that good anyway imo.
I'd switch back to Safari, and I try it out regularly, but there are more issues than I have with Firefox.
(1) no uBlock Origin
(2) no decent cookie management such as a simply delete-on-exit, let alone white lists
(3) going "back" in some forums results in a delay and a page reload
(4) Can't set custom search engines. I want my searches to always use google.co.uk regardless of where I happen to be
(5) I don't like the tab bar in either standard or compact modes as they keep resizing
(6) I typically close tabs regularly and unlike Firefox, closing the last tab closes the window. Yeah, this is minor
(7) For typical web pages, Firefox is faster, maybe due to uBlock-Origin being better than Adguard-for-Safari
I do like Safari's quick and easy sync across devices, and the shared-for-you stuff is kind of cool. I did miss keychain initially when first moving to Firefox, but I use Bitwarden now which has many advantages over keychain that I wouldn't go back even if I reverted to Safari.
tl;dr There's no perfect browser for me, but Firefox is the closest.