Tuba.
2 ideas for figuring this out-
I'd have all three instruments professionally serviced to make them the best instruments they could be. Tell (whoever does the work) basically what you said here- that you are trying to take up an instrument, and in addition to finding out what you want to play, are trying to figure out if your particular (guitar/piano/uke) is worth a darn anyway. You might have a really good piano, or an 'okay' guitar, or a POS piano and a great uke. Find out what they really are, not from investment perspective, but playability perspective. Trying to learn on a POS will drive you from it. A well setup good instrument will encourage you.
Lessons vs. youtube?
Go straight to youtube. What piano songs do you want to learn? What guitar songs do you want to learn? Spend time searching to see what's out there. Jump right in.
TON of stuff out there.
Use in person lessons too, but more as a musical therapist to give you tips to keep you on track. You will have more to interact with a teacher with if you dive into youtube first. They'll be able to give pointers and set you straight in areas you may not even aware you need help in.
Choosing a teacher- go right into a bar/venue, find someone you like, and ask them if they give/know someone who gives lessons.
I used to play guitar but- have pretty much forgotten most songs.
Lately I've been watching these videos to relearn stuff-
Back from mtv unplugged:
This is a better bowed instrument song, but wtf, why not?
Now- more bach in a piano tutorial:
I prefer all three of these as they are visual and sound.
I would learn as much as possible, present myself to the instructor, and say "Here's what I've been working on- now straighten me out"
edit - now youve got me watching this guy. Great teacher: