The one where your opponent is allowed to breathe. No, probably having to move your king in check - so annoying sometimes. Although not having to move your king in check would indubitably overcomplicate things...
IF U COULD ,WHICH RULE U LL CHANGE

Obviously none of the rules need changing, but from a beginners point of view I've yet to see why the en passant rule was brought over, to me and the games I ever seen it's only been a gimmick to fool the one who forgot about it. :P
*waiting for PGNs of GM games where en passant played a significant role*

En passant came in hand-in-hand with rule changes to allow pawns to move two squares forward from their start square (previously they could only move 1 in all situations) so logically that rule also should be removed if chess were to be dumbed down with the removal of en passant because it is too hard for some!
It wasn't considered desirable that a player could avoid otherwise inevitable conflict with an enemy pawn on the 5th rank by simply moving their pawn 2 squares instead of the previously only allowed one. So rather than not have the 2 square rule, en passant was brought in to add an extra dynamic and a 1-time possibility of still capturing the impudent pawn even though it moved two square rather than 1.
I have no wish to change any rules of chess (the game rules) personally. Good or bad they are what they are or have become over history. There are many chess variants and I would imagine rule changes to chess nowadays would just form another variant that barely anyone plays.

Hmm, I don't like how you imply that I'm stupid, but now I know why it's there. If you were to remove the 2 step leap from a pawn, that would alter the game completely, however losing the en passant would have no effect on most games, so your comparison is off the chart, as you probably know. Maybe you were angry with the topic itself, which I understand, as the rules are sacred to anyone dedicated to the game.
If you were to consider chess a war game, of course it doesn't make much sense that the enemy troops can somehow skip the opposition like that, now that you mention it.
For me, it's just such a rare scenario to use an en passant that I often forget the whole rule exists.

I didn't imply you were stupid, nor am I angry and nor did I make a comparison of anything. People wanting moves removed from chess because they don't understand them or forget them sounds like dumbing down to me. Whether that is interpreted as me saying someone is stupid is entirely in the interpretation!
En passant and the 2-square rule go hand in hand - that isn't a comparison, it is a fact. The en-passant rule plays a huge part in many games. It doesn't have to be played on the board in order to impact a game. The very threat of it carries an impact which in many cases means a 2-square pawn move is not played because en-passant would render it a bad move.
The skipping the opposition part comes more from the evolution of the pawn move. It went from only being able to move 1 square to adding the 2nd square. Implicitly that mean that it had the ability to by-pass an opposition pawn which is a very big change as far as pawn structure is concerned, whether everyone understands it or not. The introduction of en-passant made that a far less dramatic change rather than being an obscure change in itself.
I would change the rule that stops me from being white all the time and the rule that stops the night from making a gigantic leap and taking the king. Good luck beating me after that rule change!

what about.. each pawn reaching the last row must promote to the original piece behind it at the starting position. we would all love to keed the d-pawn.

the rule about saying "j'adoube" (I adjust); it should be updating to something more hip like "Yo like, I'm fixing this sh!!t"
PROBLEMS FROM RULE COME HERE !