Stability of keto-enol vs. conjugation
The NMR spectrum seems to show that the tautomeric state of creatinine is that of the keto-form (bottom structure).

If the ketone is the only functional group, the keto form is more stable than the enol form. One the other hand, if the carbon-carbon bond is part of a conjugated system, the enol form might be more stable (e.g. acetylacetone or phenol).

You also suggest either the imino or the amino tautomer. There is a Raman study that addresses this question and also raises the dependence on pH.