FRA and CDG are both large and can be very confusing. The biggest challenge those big airports have is the non-Schengen vs. Schengen and the fact that at times the two areas mix together and then filter back out. I find that piece to be both confusing (not the legality, that I understand but the layout) and also frustrating because what appears to be a simple connection from one terminal and another turns into a real nightmare.

SYD is also difficult to navigate because of the split between domestic and non-domestic -- nothing is more annoying than getting off a 15+ hour flight only to find out that you have to get on another bus, drive for 15 minutes to another terminal and then start the airport process all over again. Especially now with DL and VA no longer partners -- at least you can usually just drop your bags off at the counters right past customs and immigration

LHR it depends on the terminal, T2 is really simply to navigate -- it's just a long walk if you are flying out on UA/SQ/AC/etc. T3 where DL is also isn't bad if you are in J or qualify for Fast Track and can just funnel into the security and to the lounge.

Agree with MAD above, even if you are just flying on IB/BA it's confusing.

BKK is another very confusing airport if you aren't flying F (have been a while so no idea if they are still offering the cart services). Last time I was there was pre-pandemic (2019) flying in Y and I just remember how massive and convoluted it was to find the lounges and find the departure gates. I also remember how it took forever to get from arrival gate to the curb into my ride -- especially late in the evening with tons of random lines of people waiting for security and check-in (yes technically was not on DL, this was DL to ICN and then KE to BKK)