There are only 40 Planeswalkers to begin with, so by ranking 10 it's quite a few. They are all really powerful despite being expensive and rare. There is even a two mana costed one. I was trying to come up with the worst one, but I really couldn't. There are some that are geared for specific decks - swarm, artifacts...etc. But being limited doesn't mean much because in those types of decks, these cards are game breaking. All that said, here are my ten favorite.
There is always an honorable mention.This card is great, no question. Only costing three, she is definitely one of the most useful Planeswalkers. Her ultimate does some damage, her plus one hurts - but neither ability is truly game breaking.
10. Jace, Memory Adept
Drawing an extra card every turn for five mana is a good deal by itself. But you add the extra mill every turn to prevent scrying and then the plus one becomes pretty neat. Milling for ten in a mill deck is a great deal. The ultimate is the only thing I'm not entirely sold on. You wouldn't always want to do this. Most other walkers have ultimates that are no brainers in all decks.
9. Domri Rade
Generally useful removal with the minus two. Heavy creature decks are going to be loading up fast with her plus one. The ultimate will cause some good problems. All three abilities really depend on what you have though, it's not entirely powerful itself which is why it isn't in the upper echelon.
8. Karn Liberated
If you can cast this thing, it's going to be hard for your opponent to race. Adding four counters to exile a card is wicked. The ultimate is bananas. The casting cost is what knocks it down a peg or two though.
7. Ajani Vengeant
Let's print a Lightning Helix and one sided Armegeddon on the same card. Honestly those two abilities are crazy good. The key is getting to that point. It doesn't tap cards itself thus not necessarily ensuring the plus one will do anything useful.
6. Ral Zarek
I like Zarek's plus one slightly more than Ajani. Being able to tap and untap something seems better than not allowing something to untap.The minus two isn't as good, but the ultimate will win you the game just as easy.
5. Sorin, Lord of Innistrad
The ultimate is a game breaker(duh). The plus one is very useful, especially when used in conjunction with the second ability. Putting a 2/1 life-linker out every turn is going to be super hard to beat. Super great value with this card.
4. Liliana Vess
There are lots of five mana rares which do great things in Magic. Black often gets some nice cards, but not attached onto one. Not only can it tutor for 3 cards freely(if protected of course), it can also create massive card advantage and late game use the ultimate to just cause your opponent to quit.
3. Vraska the Unseen
The ultimate on this one is fairly strange and potentially limited. The other two abilities more than make up for it. Not only can it gain counters by deathtouch, it can also blow up any nasty permanent causing you issues. The five loyalty is great enough that it will at the minimum cause your opponent to spend great resources to deal with this.
2. Jace, the Mind Sculptor
Yes, it's not my number one. Being my second favorite Planeswalker still makes him one of the best cards in the game. A Brainstorm/Boomerang that can also choose if your opponent should keep their next draw? These are great abilities no question. I do like built in protection (8 of them on this list have some sort of it) which Jace is lacking. But if played at the right time, this card will undoubtedly take over a game.
1. Garruk, Primal Hunter
Clearly it can't be splashed, but what this version of Garruk does is no small feat for any green player. A 3/3 creature every turn is it's worst ability. Playing around 6 6/6 creatures as an ultimate is no joke. Being able to draw cards is something green just shouldn't be able to do. In two turns this thing could net a minimum Beast and three cards - for five mana is a sweet deal. One that will quickly dominate the game if not immediately answered.