Depends on what you start out with and have to work with...
Overall though, tufts are probably the easiest to add in while being extremely hard to perfect an even DT... but easiest is usually best left for last...
This is my opinion and how I see things from what I have learned about and from breeding...
In any breed, seriously working towards breeding to SOP, always focus on type first, then color, then the finer details...
So you would want to work on rumplessness first... then shape, size, carriage, and weight... then work on color... and lastly work on tufts...
Now I feel the most emphasis should be placed on your cockbird rather than your pullets... I see it over and over, most are concerned with getting pullets, better pullets, perfect pullets... while this is not wrong, I feel the more important keystone for improvement lies in the cockbird... and this is why...
Your cockbird supplies fully 50% of all the DNA contributed to the chicks you hatch... so a lower quality cockbird can slow you down much more than lower quality pullets will... so be picky about your males... many say cockbirds are a dime a dozen, throwaways, or unwanted extras... but in the reality of the concept of breeding for a better bird, those cockbirds are essential... a great or good cockbird can move you forward whether you have great, good or not-so-good pullets... the pullets hatched from him can be bred back to him for more improvement... while settling for a not-so-good cockbird and good pullets can give pullets of lesser quality than the ones you already have... but work with what you have and as long as you continue to see improvement, cull out undesirables and move forward, you are doing fine...
Also, an added note for when working on color... growing out cockbirds will show you what and how much leakage of unwanted color you may have... pullets can hide leakage, like looking solid black except have gold or silver underneath, and carry it and pass it on to their offspring... if a cockbird has leakage, he shows it and you can see it... so be prepared for not only culling cockerels, but culling pullets or hens as needed to move forward...
Just my 2 cents but hope it helps...