Well, I would think smoking would be more rude. Obviously smoking is worse for your health and takes more effort to make it happen. While farts may smell horrible, I've never heard of someone's health being affected by them.

It's easier to hide the fact that you've farted, especially if there's more than 2 people in the lift.

When someone is smoking in an elevator, I gently rap on the huge "no smoking" sign and they normally are shamed into stopping.

Or, better, if my kids are with me, I look at the smoker, then direct their gaze to my kids, then to the no smoking sign.  The last time I did this was the guy didn't stop apologizing until he got of the elevator.  (insert evil grin here)

As far as rude things to do in an elevator goes, the worst I have seen is things like people throwing their bags of trash in it, rather than the hallway where they normally throw their garbage.  Or, there was one time where a very, very drunk guy decided to take a piss in the corner.

On the good side, I once walked into an elevator while two very hot girls were going at it with each other.  When they saw me, they stopped for a moment, smiled then went right back at it.  I then pointed at the elevator camera.  They stopped for a moment, turned a nice shade of red, then went right back at it.

There are all kind of ways to be rude on the elevator, and with the prevailing lack of courtesy, many of those are practiced every day.  Not only smoking or farting, pissing or spitting, not even throwing or leaving garbage there for others to remove when they get tired of the smell.  I have seen people moving for example, and they take over the elevator for hours at the time, not allowing any one else to use it in between their trips. 

Or sometimes someone within the building has some type of alteration done, which requires material to be moved in, and debris taking out.  They also "cofiscate the elevator and you might as well forget about it.  And they not even clean it afterwards.

And how about when the elevator will not move due to overload, and no one gets out of it.  I am sure tht others will also cover many other ways to be rude inside an elevator after me, it is common, and a way of life,  here.

How about when they walk into an elevator while on their mobile phone.  Then once the door closes and they lose reception, the incessant "wei's" that follow, each one louder than the previous "wei".   Then they jam the button for their destination floor repetitively...because that will get them there faster.

Fruit, farting would be ruder, because smoking is a normal part of society here.

If you can drop a good loud one, then you can give the smokers a good look of "hey, who cares - right?"

If you're Chinese is up to it, you could then add "Well, at least farting isn't banned in here... unlike something else". And then, if you have the ability, drop another one for them to get the point

Wearing your underpants on your head would be rude, but only crazy people would do that...

stand at the elevator door faceing the inhabitants then smile turn Fart and leave.
better after a currie or something that has had time to forment.

I smoke and can't stand it when people smoke in elevators.  Especially a group of about 6 guys all puffing away.
One thing no one has mentioned is spitting.  I have seen this many times.
THIS is what pisses me off the most....pigs.

Ultimate in rude.... an elevator smoker with really bad juicy farts.

(I threw up a little just typing that.)

I agree with hug a panda but also think that it depends on whether they are a hacker creepy old smoker with violent gas hacking into his hand and touching all the buttons can you say patient zero.