Which Qur'anic tafsir is the best?

In a Nutshell: There is no overall best tafseer for everyone. The best tafseer is one which fulfils your goal most appropriately. Moreover, the Qur'an unfolds its meanings with time so later mufasireen are probably more helpful for contemporary events. Tafseer of Qur'an is the commentary or explanation of the verses of Quran. There is generally no overall best tafseer for everyone. The best tafseer is one which fulfils your goal. General Tafseer For the layperson, I would suggest a simple tafseer of scholars like al-Baydawi or an-Nasafi or al-Khaazin. In English For those who read in English only, The Ascendant Qur'an: Realigning Man to the Divine Power Culture by Imam Muhammad Al-Asi. He has a great command of English and expresses his ideas very fluidly. Ma'aariful Qur'an by Pakistani Sunni Islamic scholar Shafi Usmani is also a good English tafseer. Comprehensiveness A more comprehensive tafseer includes Jaami' al-Bayaan fi Ta'weel Aayi-'l-Qur'aan by the Persian scholar Imam Muhammad ibn Jareer al-Tabari (838–923). He quotes opinions of earlier generations with chains (isnaads) usually not present in many books of tafseer and includes linguistic and grammatical explanations to many words. Philosophical/Theological If you want a more detailed or sophisticated tafseer, al-Bahr al-Muheet by Abu Hayyan al-Andalusi would be better; or Mafaatih al-Ghayb (at-Tafseer al-Kabeer) by Fakhr al-Deen al-Raazi. Legal For someone who wishes to understand the legal injunctions in the verses better Qurtubi is helpful. Whilst his tafseer covers the usual areas of tafseer his commentary on many of the ahkam relating to the shariah are more deeper than usual tafseers. Linguistic Zamakshari is great for those wishing to gain a deeper insight into the linguistic aspects of the Qur'an. Contemporary Tafheem ul Quran  by Mawlana Sayyid Abu al-A'la Mawdudi is a more contemporary tafseer, written over a 30 year period. It is a combination of orthodox and modernist interpretation, deeply influencing modern Islamic thought. It covers debates regarding economics, sociology, history, and politics highlighting a Qur'anic perspective showing Islam provides guidance in all spheres of human endeavour. Analytical and Research This section is a little more detailed, aimed at scholars who prefer not to leave any stone unturned in relation to tafseer; I would suggest they review the above-mentioned tafaseer in addition to the following: Tafseer al-Qur'an al-Azeem authored by Abu Hatim al-Razi. (d. 327 H) This tafseer is concise but contains a large number of interesting narrations and insights you would not find in any other tafseer. The previous tafaseer seem to depend on one another, however this tafseer was only discovered relatively recently so it was unavailable to them so they could not incorporate it into their works. It is a treasure for those who search for qara'in and shawahid of causes or occasions of revelation of certain verses. Ta'weelaat Ahl al-Sunnah authored by al-Matridi (d. 333 H). This is good for rare narrations and more linguistic and theological discussions. Bahr al-Ulum authored by al-Samarqandi (d. 373 H). This is great for gathering everything that has been said on a matter including rarer opinions of tabi'een and their followers. Al-Hidayah ila Bulugh al-Nihayah authored by Maki ibn Abi Talib (d. 437 H). This was also discovered recently through manuscripts and appears in Masters and Phd dissertations. It contains a treasure of meanings elaborated on by classical scholars as well as a narrations and qara'in (clues) that are useful in detective or analytical research. Al-Tafseer al-Wasset of al-Wahidi (d. 468 H)  is more concise but summarizes the debates and adds the causes of revelation (mostly mentioned in his book Asbab al-Nuzool) as well as including other insights. Al-Tahreer wa al-Tanweer authored by ibn Ashur (d. 1393 H) is a more intellectual tafseer. It is useful in hot debates as it evaluates the views of classical scholars without shutting one's eyes to the contemporary modern society we live in. It has a useful introduction about the aims of surahs, stories, the relationships between the verses, which are Makki and Madani and so on. Conclusion There is no overall best tafseer for everyone. The best tafseer is one which fulfils your goal most appropriately. Moreover the Qur'an unfolds its meanings with time so later mufasireen are probably more helpful for contemporary events.