Galaxy only has 1 variation, the Spanish TE. Very hard to find...and there's not to much different with the usual Galaxy. However, if you picked her, and found the Spanish one, you'd be set!
Applejack has a motherload of variants...I don't think the Arena gallery is even complete at the moment...for sure it's missing Brazilian Applejack, and I believe a Columbian white Applejack. The thing with Applejack is that she has a lot of varients, so you'd have no problem finding some, but trying to get them ALL would be a real challenge.
Sugarberry/Strawberry Fair's got a fair amount of variants, I don't think there are any besides the ones that have already been mentioned. Argentine Sugarberry is a relatively easy find...you shouldn't have too much of a problem finding her. The Brazilian one is pretty rare, but if you're patient, you'll find one...I'm pretty sure a couple pop up every year. The NC/Spanish one, however, is uer difficult to find...she'd be your grail pony if you went for the Sugarberries.
So overall, Galaxy would be a hard but quick one since you only need 1 pony...Applejack is asking for a loooooong road ahead of you, and Sugarberry will require patience and money, but is definitely an achievable goal in a relatively short amount of time.
I'd rank them as such, from easiest to hardest:
1. Galaxy...easy...no telling when the Spanish TE will pop up, so this one could literally be done in an instant!
2. Sugarberry-as mentioned, Brazilian and Spanish are pretty htf...but I think a complete family of Sugarberries could be achieved in just a few years of searching. All depends on whether a Spanish one will show up for not...but I know I've seen 1 here and there, so I think 2-3 years might even do it. Maybe less!
3. Applejack-good luck finding the Spanish piggy pony...egads...and the Columbian white version?? I've never seen either of those 2 for sale...