"Ruta's definitely easier. I know a lot of people who consider Vah Nabooris to be the hardest divine beast, because of the boss at the end.
Most people go to Ruta first. It has a pretty involved and fun quest line, in my opinion, and the rewards for completing it are definitely very helpful.
For taking on either one - make sure your inventory has enough supplies - weapons/shields/bows, healing food, and maybe try to catch some fairies around the fairy fountain at Kakariko before you go. Get a lot of arrows. Arrows are super useful. Wear your best defensive armor so you can live long enough to heal yourself.
For Nabooris and the Gerudo - you'll definitely find the Shiekah outfit helpful because a part of the questline here will involve stealth. Also get some rupees ready to buy some helpful clothing that you'll need to get during a quest there. The boss at the end of Nabooris is pretty tough, so if you go for it practice your flurry rushes - they'll probably come in handy.
For Ruta and the Zora - pretty straightforward, actually. One quest will lead you past a dangerous enemy, but with stealth you can sneak by without having to fight. Several of your runes will definitely come in handy in multiple ways - I won't spoil how, you should try to figure it out yourself when you get there - but it helps out a lot."
This is the wrong place to open the thread by the way.