There is no such “best” Upanishad, there are different degrees of penetrability.

They were never composed as bed-side or airport reading material. They are terse, recondite and need extensive commentaries to be able to understand them.

Some are more readable than others but all require commentaries or even better still a Vedānta guru.

I once attended a series of lecture on the Kaivalya Upaṇiṣad in which the Swamini expatiated on each verse for nearly 2 hours.

Vedānta is not a DIY learning project - it requires many hours of intensive study and memorisation - powered by inquisitiveness, determination and dedication.

As is said in one the Upaṇiṣad:- na ayam ātma bala-hīnena labhya - This knowledge of the Self cannot be obtained by the weak.

All learning (according to Hindu epistemology) should be directed towards self-transformation and application - and to actually change and create change around you, requires immense strength of character and courage. If this is lacking then best not to even attempt to dive into the Upaṇiṣads.

As the Kaṭha Upaṇiṣad says - this is the path of the Razor’s Edge! (kṣūrasya dhārā)