All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There is no doubt that it is more appropriate to recite in the Witr prayer the supplication taught by the Prophet to Al-Hassan Ibn 'Ali .
The supplication is as follows: "O Allaah, guide me along with those whom You have guided, pardon me along with those whom You have pardoned, be an ally to me along with those whom You are an ally to, and bless for me that which You have bestowed upon me. Protect me from the evil that You have decreed, for verily You decree and none can decree over You. For surely, he whom You show allegiance to is never abased and he whom You take as an enemy is never honoured and mighty. Blessed and Exalted are You, our Lord."
The Arabic text of the above Qunut supplication is as below:
(اللهم اهدني فيمن هديت، وعافني فيمن عافيت، وتولني فيمن توليت، وبارك لي فما أعطيت، وقني شر ما قضيت، إنك تقضي ولا يقضي عليك، إنه لا يذل من واليت، ولا يعز من عاديت تباركت ربنا وتعاليت)
[Allaahumma ihdinee feeman hadayt, wa 'aafinee feeman 'aafayt, wa tawallanee feeman tawallayt, wa baarik lee feemaa a'tayt, wa qinee sharra maa qadhayt, innaka taqdhee walaa yuqdhaa 'alayk, innahu la yathillu man waalayt wa la ya'izzu man 'aadayt, tabaarakta rabbanaa wa ta'aalayt].
You may also supplicate with other supplications that are reported in authentic Ahadeeth and which are mentioned in Fatwa 81651.
However, if a person does not know such supplications, he may say any supplications as some scholars are of the view that there is no particular supplication to be said in Qunoot and that the Companions are reported to have said various supplications.
Allaah Knows best.