What kind of poker would be hardest for loose pre-flop amateurs to play? Any real game such as 7-stud, PLO-4, PLO-5, Courchevel, 3-Draw, etc, along with made up ones would be accepted.
A made up game could for instance be PLO with an additional street and betting round, or to play PLO with two boards and after the last betting round you take all board cards shuffle them up and lay them out randomly.
This is a matter of psychology and thus behavior of weak players and not about opinion.
If there is a toss up between all types of poker there is and isn't then this would be a matter of opinion. Otherwise there will be an answer that is better then the next one. Or correct me if I'm wrong.
I don't know if the rules allow adding an answer into the question but I'll try that and can edit this later. From my experience it is very good to play two board games since bad players are satisfied to play one of the boards and thus get freerolled a lot. Besides that I'm in vacuum. Some games get tighter when you play 5 or 6 card PLO while some get looser.