Atheists, what are the most convincing theist arguments you heard of?

[Nonpareil]

The Kalam argument is nonsensical.

I understand the sentiment because it is a poorly constructed version of the Prime Mover, the First Way of Thomas Aquinas. As such it appears to beg the question, by stating a condition for beings (coming into being) then seems to make an exception for a being with the unique feature of not having come into being.  The missing premise that clarifies the demonstration is the continuum of existence from actual being to potential being.

For the purposes of comparison, the Kalam cosmological argument:

1. Everything that has a beginning of its existence has a cause of its existence.
   
2. The universe has a beginning of its existence.
   
3. Therefore, the universe has a cause of its existence.
   
4. If the universe has a cause of its existence then that cause is God.
   
5. Therefore, God exists.

And the First Way of Thomas Aquinas:

1. It is certain, and evident to our senses, that some things are in motion.
   
2. Now whatever is moved is moved by another.
   
3. If that by which it is moved be itself moved, then this also must needs to be moved by another, and that by another again.
   
4. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover, seeing that subsequent movers move only inasmuch as they are moved by the first mover: as the staff moves only because it is moved by the hand.
   
5. Therefore it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God.

Both ultimately fail because their basic premise is bare assertion. The need for a cause can only be established to hold within the universe itself; asserting that there is a need for a cause for the universe itself is an unsupported assertion, and edges towards the fallacy of composition. This is entirely without getting into any issues with the specific formulations and the vocabulary that they choose to employ, and holds across every variation of the cosmological arguments. They may also get into other fallacies, like circular reasoning, but none of it actually matters.

Because the central premise is unsupported.

The argument is, thus, discarded.