Ecumenical Councils are the top of the list, simply because the whole of the Church gets together to promulgate doctrine. These doctrines are signed by the Pope at the end of the Council, and are binding. Some of the doctrines are administrative in nature, and can be modified, changed, added to, or removed. Others, which deal with faith or morals, are eternally binding. An example of this would be in the Council of Trent, where the Catholic Church defined the books of the Bible explicitly (they had done so in the past, but they chose to reiterate it because Luther was removing books), faith vs works, etc., in response to the Protestant Reformation. Those decisions are binding and infallible.
Encyclicals are next on the list, because they deal with specific matters of faith. An example of this would be Humanae Vitae, the encyclical which banned birth control from use in the Catholic Church. Again, that teaching is binding. When the correct wording is used, they also become infallible, such as Ineffabilis Deus, which defined infallibly the Immaculate Conception:
Wherefore, in humility and fasting, we unceasingly offered our private
prayers as well as the public prayers of the Church to God the Father
through his Son, that he would deign to direct and strengthen our mind
by the power of the Holy Spirit. In like manner did we implore the
help of the entire heavenly host as we ardently invoked the Paraclete.
Accordingly, by the inspiration of the Holy Spirit, for the honor of
the Holy and undivided Trinity, for the glory and adornment of the
Virgin Mother of God, for the exaltation of the Catholic Faith, and
for the furtherance of the Catholic religion, by the authority of
Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by
our own: "We declare, pronounce, and define that the doctrine which
holds that the most Blessed Virgin Mary, in the first instance of her
conception, by a singular grace and privilege granted by Almighty God,
in view of the merits of Jesus Christ, the Savior of the human race,
was preserved free from all stain of original sin, is a doctrine
revealed by God and therefore to be believed firmly and constantly by
all the faithful."
Hence, if anyone shall dare -- which God forbid! -- to think otherwise
than as has been defined by us, let him know and understand that he is
condemned by his own judgment; that he has suffered shipwreck in the
faith; that he has separated from the unity of the Church; and that,
furthermore, by his own action he incurs the penalties established by
law if he should are to express in words or writing or by any other
outward means the errors he think in his heart.
The bold would be what makes it infallible. Another Pope could not undo this doctrine, as it is an infallible statement on a matter of faith. Note too that it is binding on ALL the Church as well, as stated in the last paragraph of the quote.
An Apostolic Exhortation is more of a letter of encouragement. It too teaches, but they tend not to impose a whole lot on the faithful. Rather, they are written to encourage holiness and growth as a Christian.