Another vote for tires. The thing that keeps you in contact with the terrain you're riding makes a Huge difference. After that, I say assuming it's a bike that already fits you well and functions well - the fork if it's a bike with a suspension fork. If fully ridged then I'd vote breaks.

Tires

tires are a really good pick but they're also fairly subjective. there are a FEW tires with a very solid reputation that are widely loved, but there are many other tires that depend on your specific riding conditions and your own preferences.

so that's what sucks-- we can unequivocally say that at a given price point, this one/two fork(s) is the best, but it's nowhere near that easy with tires.

Let's go ahead and open it up to specifics.

Lots of folks have said tires, but which would you pick?

("specifics" doesn't limit the discussion to tires, alone. If there's a particular fork, brakeset, etc. that you feel makes the biggest difference, feel free to say so.)

What are you riding? Tires are very specific to conditions.

I have to say that the (brand) (model) in (size) of (durometer) is the perfect tire. Except when it's not.

i'm still pretty new to mtb but i figured all you guys would say change the fork first. tires wasnt something i expectd to read. my jet9 came with conti xking tires on it, i got a hole in the sidewall so i ordered some maxxis ardent tires to replace them. my buddy that rides them said they rail corners. i'm hoping they make a big difference, i need all the help i can get. i'm just now learning i need to put some weight over the front tire instead of leaning back. now it seems like a no brainer.

If people are going to post tires, brand and size can you also post typical riding conditions as well?

A typical ride for me starts with several miles of pavement, which gives way to several miles of gravel, which gives way to several miles of hard pack. The hard pack can have miles of relatively soft dirt and some loose sand. Throw in some mud and ruts. My Rockhopper came with Maxxiss Capitan studded tires or something like that. I switched them out for some cheaper Avernir Swiss Army universal tires I got from Dick's Sporting Goods for $17 ea and I couldn't be happier. The Avenir tires are much much faster on smooth surfaces and they grip well on dirt. The only places they slip more than the studs are on the loose sand or deep mud, and those are places I try to avoid anyway.
Maybe now I'll look into upgrading the fork.

helium.

Since tires, wheels and all of that has been mentioned. A comfortable set of handlebars.