Every map is challenging with less amount of people in general (tbh mostly only when new guys were around, not regulars). Like Outpost STD for example, that's one of the map that has BLUFOR tickets lower than the REDFOR, so playing the long game for 2.5 hrs is not possible to win by ticket count.
Another example would be some Vietnam maps, such as Vung Ro ALT. Same case as previous map above, but you play as the OPFOR and a way lower ticket count against BLUFOR side.
Some others are night layouts, such as Ia Drang ALT layout is challenging, if you don't set your gamma max since it's very dark ngl. Black Gold INF and Tad Sae Offensive ALT also fell off to this case. There used to be Sbeneh Outskirts ALT which also plays as OPFOR and night layout, but no longer there anymore as it's an entirely new layout in daytime.
There's a lot more, but these pretty much that stands out to me the most from previous experience and recent.
- Inch