A lay disciple asked Geshe Potawa which Dharma Practice was the most important if one had to choose only one. The Geshe replied:
If you want to use a single Dharma practice, to meditate on impermanence is the most important.
At first meditation on death and impermanence makes you take up the Dharma; in the middle it conduces to positive practice; in the end it helps you realise the sameness of all phenomena.
At first meditation on impermanence makes you cut your ties with things with the things of this life; in the middle it conduces to your casting off all clinging to samsara; in the end it helps you take up the path of nirvana.
At first meditation on impermanence makes you develop faith; in the middle it conduces to diligence in your practice; in the end it helps you give birth to wisdom.
At first meditation on impermanence, until you are fully convinced, makes you search for the Dharma; in the middle it conduces to practice; in the end it helps you attain the ultimate goal.
At first meditation on impermanence, until you are fully convinced, makes you practise with a diligence which protects you like armour; in the middle it conduces to your practising with diligence in action; in the end it helps you practise with a diligence that is insatiable.
And Padampa Sangye (a famous Indian siddha 11th-12th century) says:
At first, to be fully convinced of impermanence makes you take up the Dharma; in the middle it whips up your intelligence; in the end it brings you to the radiant dharmakaya.
Unless you feel this sincere conviction in the principle of impermanence, any teaching you might think you have received and put into practice will just make you more and more impervious to the Dharma. He also said:
I never see a single Tibetan practitioner who thinks about dying;
Nor have I seen anyone live forever!
Judging by their relish for amassing wealth once they don the yellow robe, I wonder-
Are they going to pay off Death in food and money?
Seeing the way they collect the best of valuables, I wonder-
Are they going to hand out bribes in hell?
Ha-ha! To see those Tibetan practitioners makes me laugh!
The most learned are the proudest,
The best meditators pile up provisions and riches,
The solitary hermits engross themselves in trivial pursuits,
The renunciates of home and country know no shame.
Those people are immune to the Dharma!
They revel in wrong-doing.
The can see others dying but have not understood that they themselves are also going to die.
That is their first mistake.
Meditation on impermanence is therefore the prelude that opens the way to all practices of Dharma.
(Extracted from Words of my Perfect Teacher)