Current game i'm trying to 1cc is Willow (up to final stage on 1 man) isn't that hard.
Games I can remember (1cc - again, not that hard...or maybe not back when I played some of them)
Gradius (NES) 1 ship
Mike Tysons Punch-Out inc Mike (NES) (1 man)
Super Mario Bros (NES) All levels (1 man)
Double Dragon (arcade) (1 man)
Golden Axe (MD) (1 man)
Son Son II (PCE) (1 man)
Shinobi (arcade) (1cc)
Trojan (Nes) 1 man
Gunsmoke (nes) (1cc)
There are others, but the above were challenging, Mike Tyson took months of practise, same as super Mario Bros & Gradius.
Double Dragon, Golden Axe and Son Son II were matters of weeks or less.
Loads more, but non worthy of mentioning.
Ninja gaiden nes on 1 credit 1 man!! man, it took me weeks with infinite continues...thats one game I didn't even try to complete on 1 credit, although...maybe one day.!
EDIT
added a couple more, the nes games were largely played in the 80's/e 90's, i may of only had a few games, so they became natural with playtime. As i mentioned Willow, its 3 weeks old in my collection and i'm specifically trying 1cc then 1 man. I'm on last stage, but i can still lose a man on several parts, stage 2 sometimes gets me more than most...i'm enjoying it tho.