Other than proving that you're skinny, a six-pack isn't much good for cycling. Ever have sore abs from cycling? I've never heard of it, but it you do, then ab work is probably good. Otherwise, it's your posterior chain that gets a workout on the bike, and your back is the upper anchor for that. Sore back complaints are legion here at BF. I do 5 times as much back work as ab work.

I like, in no particular order:
Lat pull-downs w/arched back
Horizontal row
Bent-over dumbbell rows
Dumbbell deadlifts
Back machine
Barbell squats
Seated unsupported dumbbell presses

The only thing I do at the gym that's really ab-centric is leg lifts in the Roman chair. Note how her knees come up above her hands: https://www.youtube.com/watch?v=3dbUfm0FC4I
This particular exercise imitates pedaling. One-legged pedaling also works the lower abs.

For bodyweight stuff, I still like Core Advantage.