For me, changing handlebars on a bike changes the bike's character more than anything, because it affects rider position so dramatically. My Super Course came to me with drop handlebars. I've had all-rounder (slightly bent upright) bars, forward bending bars, north road bars on it. For a while, I flipped the north roads upside down, but I didn't like that. Currently, it has all-rounders with bar ends added.

I can't find a picture of the bike with the drop bars.

It currently has these all-rounders.



But as I said, it has bar-ends on it now.



Before that, it had these weird so-called Ergo bars.



And before those, it had these aluminum north roads, which I regret I installed on a bike I sold.

Tires.

And accordingly it is the place where I am most willing to spend a lot.

I'll list saddle, after tires. Crankset, after these.

I second this vote.

First thing I thought of was wheels/tires as to the feel of the ride. I have done more handlebar changes than any other thing though, so I think that handlebar changes necessitate a saddle change or at least an adjustment. For example, right now I have a Brooks B67 on my mountain bike drop bar conversion (1995 Trek 820), because it's the only saddle I have at the moment that is "free". I definitely feel & know that this is NOT the right saddle for me on this bike, and it will be changed once I acquire another black Brooks B17. However, if I had different handlebars on this bike I could likely use this saddle, so again....boils down to handlebars. Interesting topic. Interested to see what others say.

1. Handlebars, freewheel/cassette
2. Wheels, tires, saddle (I'm fairly insensitive to saddles)
3. Stem (if very different size), chainrings/crank (ditto)
4. Bar tape (just because you have to look at it)

1) Physical fitness.

Seriously, go ride a lot, lose some weight and you'll notice all of your bikes seem better in so many ways.

2) Tires.

Don't be a cheapskate.

Let me see if I have your order right.

1. tires
2. saddle
3 crankset

Now how does a crankset change become a big one?

This 52 & 49 half-step gear range was useful when I owned this bike in 1975;



This 52, 42 & 30 gear range is better for how I ride today;

I think it is a matter of what your asking the bike to do. As Noglider himself notes changing the style of the handlebars will drastically change the character of the bike and how you intend to use it.

If yure simply looking to get more performance out of a bike I was always taught that changing the wheels and tires gave the best upgrade for the money.

For things like changing the size of the bars and stem or saddle have more to do with making the bike more comfortable than performance.

At one time and age, I would have said wheels. These days, at my age, I am 100% with noglider - handlebar choice, hands down (pardon that not intended near pun). My Motobecane Gran Jubilee has gone from drop bars, to mustache bars, to upside down mustache bars, to flat bars and finally, the bell ringing north road, or something very similar.

Yup, for me, it is handlebar choice, coupled with an appropriate steering stem reach...

Agree with all mentioned so far. Here's my list in order of most significant to lesser so (leaving out stuff like aero bars and time trial helmets, etc):

Tires (high tpi natural casings seem the fastest)
Wheels (lower spoke count is effective in reducing drag and steering responses due to cross winds), with smooth hubs
Tubes (latex seems to roll easier than butyl ones, not sure why)
Chain (keep in top working order and replace often)
Chain-line (not a part, but cross chaining just seems to eat power)
RD - a smooth working RD saves time both in shifting and runs better all around
Rings - some rings (SRAM in particular) just seem to run crappy for me, whereas something like a TA ring runs noticeably better
BB
FD - more in terms of adjustment than the part itself

Saddle of course will occupy a place on the list depending on your tolerance for pain in your nether regions. Same thing for handlebars, if you can stay in the drops longer you'll be faster.

@Barrettscv, thanks for clarifying. I was picturing a replacement of one crankset for another with the same gearing. A few people can feel that, but it's got to be subtle.

@Bianchigirll, very good point. Tires and wheels are usually the best upgrade, but handlebars can change the character of the bike. And of course, when you change handlebars, it is often necessary to change the stem or seat or both. Or at least, you need to readjust. Going from drop bars to upright bars is likely to require a wider seat or tipping the seat up at the nose.

Cant argue with that! I notice the difference for every 5lbs lost. Tires? Yes, but for me it also is about weight.

Then there was that set of light racing wheels I put on.

The red bike reminds me of a Mondonico a LBS owner has as wall art.

Not sure if the question relates to what changes the look most dramatically, or the feel. Unquestionably, the feel changes most with tires/wheels for me. I have to agree that changing the style of handlebars changes the look significantly. As a back up answer though, I find that the addition of fenders has a surprisingly dramatic change to the overall character of a bike.

I'm going to say that the drivetrain and shifting mechanism make the most difference. If shifting is easy like for example Deore thumbshifters on a mountain bike, you don't really think about the best gear ratio for any given riding condition, it becomes just an automatic reaction to the load on the pedals.

For me, the biggest bang for the buck is a new chain. A brand new one can make even the humblest crank feel lively and responsive.

tires
seat
cables (If your existing cables are old school...)

legs

I agree.

I have been losing weight and gradually improving my conditioning over the last nine months.

I am by no means fast anymore, but I'm not afraid of long climbs for the first time in a decade.

Wellness is my goal and these bicycles help me meet my goals.

Back in the day, we couldn't afford a great bike, but our next purchase after the bike was some better wheels - usually sew-ups. The improvement the tubulars made was generally dramatic. Not always true nowadays, since clinchers have improved so much, but back then the upgrade was wheels. And you could sell them later if you lost interest.

Yes, I put straight bars on this old cleaned up Bott professional and it's great for commuter rides. They made the bike feel so different

I would add that when rehabbing a vintage bike, I notice performance improvement from hubs that have been cleaned, serviced with new bearings, and properly adjusted. It pays to have great hubs to begin with, but after they have been serviced, the efficiency of the bike improves a lot. During a Detroit Slow Roll, I find I don't need to pedal as much or fast to stay with the pace among hundreds of vintage bikes that clearly look neglected maintenance wise.

Tire pressure influences pedaling efficiency as well.

For me:

1-Wheels, be they new, trued, tensioned, repacked, whatever. It's what I feel.
2-Crankset/BB. For some reason, I can tell a difference down there. Not sure what, but I notice.

I don't change bars, pedals, stems, frame size.
I don't really care about saddles, wrap, calipers, FD/RD or shifters, as long as they work.
Sometimes, I feel a tire crawl, and it bugs me, but then again, I also think my brakes rub about 50 miles into every ride.

So, in that order: Wheels, then crankset/BB.