In my Ottoman game I used marches VERY extensively - to the point of having 4 of them sometimes, and for 200 years I had 3 of them - and here are my experiences.
WALLACHIA!
Seriously, these guys as a march were more badass than most of independent countries in my game. Their NI set + march status allows them on having 10 - 15 000 good quality troops with good general, and generaly they can create surprisingly big mess for the enemy.
Not to mention the fact that they have +100% hostile creation cost + mediocre income so you shouldn't annex them anyway. Also, their strategical position is cool: just between Ottomans/Byzantium, Lithuania, Poland and Austria/Hungary. Very useful for dominating Balkans
GEORGIA!
Next top quality march which also - for some weird reason - looks really cool on the map. I mean, they have very nice colour. Their troops have worse quality than Wallachian, but they recompensate it with more provinces/better income so they field army enough big to help significantly. Also their terrain is mountains + hard to siege + often some sillier AI armies go to siege Georgia and can be exterminated easily.
Again: +100% core creation cost and Orthodox make them not worth to annex, while they are very useful march due to being between Ottomans, Timurids, Hordes and Russia.
CROATIA
'wait, what, Croatia?'
Yes, because Croatia lies in this perfect place between Italy/HRE and Balkans. It was extremely useful for my Ottoman Empire as help against HRE/Italian coalitions, but I can imagine being very useful for them in the same way. Also Croatia is able to maintain decent fleet and block invasions on Balkan/Italian coast from the other side.
Plus it has relatively small culture, so there are limited chances of you being able to tolerate it anyway.