What's better than crunches? What is the best exercise to for Abs?

If you’re looking to get the most bang for your buck for burning calories, then you want to perform exercises that are metabolically demanding. Usually, the exercises that we hate the most, the ones that leave you gasping for breath, are the ones that burn the most calories. Doing bench press, is not going to burn a lot of calories despite it being guy’s favorite exercise to perform. Most people would think that the squat or deadlifts would be the most metabolically demanding exercise to perform, but the newest research may shock you. Researchers wanted to quantify and compare the acute metabolic responses to resistance exercise protocols comprising free-weight, body-weight, and battling rope exercises. Ten resistance-trained men performed 13 resistance exercise protocols on separate days in random order consisting of only one exercise per session. For free-weight exercise protocols, subjects performed three sets of up to 10 repetitions with 75% of their one repetition maximum. For the push-up and push-up on a BOSU ball protocols, subjects performed three sets of 20 repetitions. For the burpee and push-up with lateral crawl protocols, subjects performed three sets of 10 repetitions. For the plank and battling rope circuit protocols, subjects performed three sets of 30-second bouts. A standard 2-minute rest interval was used in between all sets for each exercise.

At the end of the study, oxygen consumption was significantly greatest during the battling rope and burpee protocols. For the free-weight exercises, highest average values were seen in the squat, deadlift, and lunge. No differences were observed between push-ups performed on the floor vs. on a BOSU ball. However, adding a lateral crawl to the push-up significantly increased mean oxygen consumption. The lowest mean value was seen during the plank exercise. These data indicate the performance of exercises with battling ropes and a body-weight burpee exercise elicit relatively higher acute metabolic demands than traditional resistance exercises performed with moderately heavy loading.

Battling Ropes Exercise
Here are a list of the exercises:

Burpee

Planks: Best Abdominal Exercise
Another interesting finding was reported in the Journal of Strength and Conditioning Research was that crunches may not be the best exercise for abdominals. In a previous study that compared abdominal and erector spinae (lower back) activity during exercises with and without a balance component. For example, the participants completed a bridge exercise with both feet on the ground and with one leg lifted. The average EMG amplitude was also at least 20% greater in the abdominal muscles with one leg elevated and was 200% higher in the external oblique muscles. This means exercise that requires balance activate the abdominals more than stationary abdominal exercise. Countless exercises target the primary core trunk muscles (abdominal and lumbar) with the aim of providing these benefits. However, it is unknown as to which exercises elicit the greatest activation thereby maximizing functional gains and peak performance. Researchers examined whether integration core exercises that require activation of the distal trunk muscles (deltoid and gluteal) elicit greater activation of primary trunk muscles in comparison with isolation core exercises that only require activation of the proximal trunk muscles. The participants then performed 7 core exercises (4 isolation and 3 integration, respectively): the crunch, an oblique crunch, prone back extension with forward arm elevation, bird dog with resistance and hover with contralateral arm reach, side plank with arm raise, and mountain climber with alternating hip flexion to the opposite elbow.
Here are a list of the exercises:

At the end of the study, the researchers found that the activation of the abdominal and lumbar muscles (lower back) was the greatest during the exercises that required shoulder and gluteal recruitment. In fact, the abdominals and lower back were generally 20% greater during integration exercises (i.e. plank exercises) compared with the isolation exercises (i.e. crunches).
In conclusion, when completing abdominal exercises, including planks and other exercises can enhance the abdominal activation of those muscles compared to traditional crunches.

Ratamess NA, Rosenberg JG, Klei S, Dougherty BM, Kang J, Smith CR, Ross RE, Faigenbaum AD. Comparison of the acute metabolic responses to traditional resistance, body-weight, and battling rope exercises. J Strength Cond Res. 2015 Jan;29(1):47-57

Gottschall, Jinger, Jackie Mills, Bryce Hastings. Integration Core exercises Elicit Greater Muscle Activation than Isolation Exercises. Journal of Strength and Conditioning Research.