Should you invest in 1 corporation, the one you deem most effective? The investment to pay-off scenario matches that of charitable giving rather well, except you are the beneficiary (and you do care not to invest in something that flops over and goes bankrupt)

You have strong reason not to do this anyway because of risk aversion. This is like saying, "Should you serve butter or margarine to your guests? To get a better intuition, consider the selfish version, where you are yourself going to eat either pristine butter, or a container of margarine that has been poisoned with arsenic?"

If you assume that your algorithm for choice of charity might be faulty in an exploitable way, the #1 charity may be sufficiently able and motivated to exploit you - having all your money as reward (and money of anyone who's reasoning like you) - but all the top #5 , five times less so.

I agree this is an issue, and that you should take manipulable signals as weaker evidence because of Goodhart's Law. But this effect doesn't automatically dominate. Selecting for good expected value with your best efforts incentivizes efforts to produce signals of value, through real as well as fakeable signals.

Note that GiveWell and friends do not follow your heuristic: the great majority of funds flow to the top charity. They take into account the possibility of faked data (to mess with CBA) in their evaluation process, valuing independent verification, defenses against publication bias, audits, and so forth. But in light of those efforts, they think that the benefits of incentivizing (and assisting) more effective and transparent charities outweigh the risk of incentivizing fakers who can defeat their strong countermeasures.