Normal. Challenging but stillquick to get through the story.
Then maybe up a lvl depending on what changes. e.g ME2 adds shields and armour mmaking biotec powers useless, so I gave up on anything with those in.
Fallout NV just makes things more annoying, my legion character had a machete and tried to kill a fiend, by the time 1 fiend was dead it was half broken... [censored] that, it souldnt happen at alll. I would be fine with HC mode if it werent for companion deaths, because cazador poison kills them because they cant use antivenom. NV styas normal.
Only game I bothered completing on hardest was halo reach. Thats because I wanted the "I am a monument to all of your sins" but it was kinda doable it wasnt the missions that were the problem, just parts of the missions, like the last mission, I found killling the brutes quite fun, be the overshield zealots went to far (skipped past hunters

). The only 3 missions I hated with a passion were long night of solace, nightfall and the pillar of autumn. But aside from that I found it quite fun.
I also like RDR it dosent make enemies bullet sponges with sniper rifles, just removes the aim assist, I enjoy it in single player, much more fun, but balenced, a couger till dies in one shot, undead nightmare however I like casual (damn zombie cougers) so it depends on the game really. If your idea is to make extremely unbalenced gameplay e.g Fallout NV/3 then no, I am not going to find shooting them in the head 50 times to kill them and them shooting me once in the chest balenced. Should just be everyone dose huge damage, headshots are a 1 hit kill, you them, everyone. But if its done in a semi balenced way (RDR ) then sure why not.
I still think AI should improve with difficulty instead,