Adlerdriver

"Challenging" can be very much in the eye of the beholder. Looking at 777 trips, one would have to begin by assuming spending 13-15 hours in-flight regularly is acceptable. Some guys would never even consider that, so it really depends on perspective. Beyond that, single leg duty periods were always a goal of mine. I might be willing to accept a two-leg day on a 12-14 day trip to get an international front-end deadhead on the trip. With reasonable seniority, it's not difficult to avoid turning.

777 trips that have multiple leg duty periods usually involve an initial long flight to Europe or Asia (or maybe a deadhead there to start). After that, some of the city pairs I already mentioned with 1-2 hours on the ground between legs. Times are usually "night" in the local area but the CGN-CDG-MUC-FRA duty period is during the "Euro" day, ending up in FRA in the early evening. Some guys might make a big deal that the "night" flying in Asia is really day flying on their body clock. I've never cared. Once I raise the gear on leg #1 and leave the US, I don't care what time it is back home. Putting that in my cross-check is a sure fire way to screw up otherwise good rest. Sleep when tired has always worked well for me.

The most unpopular 777 duty periods for me are the 7+ block hour, single leg, 2-pilot flights. DXB-SIN, SIN-KIX, SIN-SYD followed very closely by the MEM-ANC or IND-ANC flights. I'd rather bang out a couple of 1-2 hour legs with a quick turn than do those longer, drool cup flights with 2 guys. But, there are some challenging two leg days on the 777 too. I'll give you a few examples of duty periods I would avoid with city pairs along with the takeoff / landing times in Z. You can do the math if figuring body clock time is valuable to you.

HKG-KIX-NRT (1420-2140) - 5:14 Block
NRT-ICN-SZX (0005-0800) - 6:08 Block
(the two DPs above are back to back on the same trip with a 24 hour layover - i.e. a complete circadian swap required on that layover)

NRT-PEK-PVG (1225-1920) - 6:12 Block
Any turn thru PEK has huge suck potential because of ATC delays and the incredible ability of local controllers to turn a 10 minute taxi into a 1-hour Magellan tour of the airport.

Here's a completely different ball park. A 3-duty period series of hub-turns CDG-CGN-CDG. 3 days in a row, takeoff at 18:50Z, land back at CDG 22:46Z. So, basically day flying considering a US body clock. 2:27 Block each day. 17-hour layover between each duty period. If it was a MEM domestic trip to ATL and back, it would go senior.
Now the rest of the story - The trip starts a week before these 3 duty periods and goes MEM-ICN, then ICN-HKG, then HKG-LGG followed by a deadhead from LGG to CDG. If you still know or care what your home body clock after US-Asia, intra-Asia and Asia-Europe revenue flights, you're much better at sleep management than I. So, I would consider this trip challenging when the details are considered.