First of all you need to make sure that your body fat percentage is low enough to be able to see the definition.
I was doing crunches and sit-ups for a long time and didn’t see any improvement - then found that my body fat percentage was too high. It also depends on what extent you wish the definition to be. Bodybuilders have low percentages like 4/5. But if you have around 8 - 10 percent then you should be able to see them.
If it is too high, you’ll need to perform lots of cardio/aerobic exercises and eat healthily as to bring it down.
When it’s at around 8 - 10 percent, weights are probably the best for arm muscle definition.
Then lots of crunches and other anaerobic exercise to define the abs. One of the best pieces of equipment for abs are fitness balls, as they allow the crunches to be much more effective.
Using a combination of the treadmill with weights is fantastic for strengthening both arms and legs, and is also thought to be the best form of Cardio Exercise so I would reccomend (after lowering your bf%) Sit-ups and Crunches for the abs, and the Treadmill combined with weights for everything else.
Good Luck.

i use a pilates/ yoga band when doing pilates. ive noticed my stomach getting flatter in a week. there is also the fitness ball, which i dont use, but i hear is good. and also like a fitness rope, it has handles on the ends, i dont know the exact name of the thing, but it came with a weight loss program my mom got into. best thing, in my opinion, pilates band.