Planks are objectively the best exercise for overall core strength, but only if youre doing them correctly. If youre not contracting the abs then youre just using your shoulders and hip flexors to stay up, and obviously thats not what you want. The biggest downside to planks is that holding one position gets boring as you get stronger. So you should experiment with different types of plank exercises once youve developed enough to stay raised for a minute with no issues.