What is more rewarding, doing Zikr such as saying Tahleel and Takbeer or reading the Qur'an?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There is no doubt that the best form of Thikr (remembrance of Allah) is reading/reciting the Quran, and it is greater in reward than all forms of Thikr such as Laa Ilaaha Illa Allah, Allaahu Akbar, and so on.

The following points prove the above-stated opinion:

i) Quran is the words of Allah, The Exalted.

ii) The Prophet said: “Whoever recites a letter from Allah's Book (Quran), he will be credited with one good deed, and a good deed gets a ten-fold reward.” [At-Tirmithi and others]

As for other forms of Thikr, one will be credited with one good deed for every Tasbeehah (i.e. saying Subhaana Allah: Glory be to Allah) and so is for every Takbeerah (Allahu Akbar) and every good deed will be with a ten-fold-reward.

Abu Thar narrated that the Prophet said: “Truly, each Tasbeehah (saying Subhaanah Allah; i.e., glory be to Allah) is a charity; each Takbeerah (saying Allahu Akbar; i.e., Allah is the Greatest) is a charity; and each Tahmeedah (saying Al-Hamdu lillah; i.e., All praise is due to Allah) is a charity; and each Tahleelah (saying Laa Ilaaha illa Allah; i.e., None has the right to be worshipped but Allah) is a charity.” [Muslim]

Sa'd ibn Abu Waqqaas narrated that the Prophet said: “Who is incapable among you to do one thousand good deeds everyday?! One asked the Prophet (): ‘How can one of us earn one thousand good deeds everyday?’ The Prophet () replied: “If one says Subhaanah Allah one hundred times, he will be credited (by Allah) with one thousand good deeds or one thousand misdeeds of his record will be forgiven.” [Muslim]

So, if one says Subhaanah Allah, ten good deeds will be recorded in his account with Allah, although the word Subhaanah Allah includes ten letters; while if one recites ten letters from the Quran, a hundred good deeds will be recorded in his account.

Allah knows best.