Agree with @LieutenantCommanderData that Swap is my favourite card aesthetically, but I also love some of the flavour text: Fireball Tricky Stab Firestorm Toughness Strong Bash There are lots and I could list about 20, but these are some of the best.
Shambling Bash I always thought that a jelly got flipped into the air when I saw that one. Feel free to pick your favorite flavor text too.
My favorite flavor text I can think of is Draining Touch. I haven't thought much about what illustrations I like, but Mind Muddle and Winds Of War are rather pleasing. On a bit of a tangent, I think more powerful-looking artwork subconsciously leads me to pick cards over others more often in Quick Draw; similarly, I tend to just assume that dwarf warrior with a helmet covering her face has armor.
There's just something cool and badass about Nimble Strike, both art and flavor text are great. Be like the wind. The wind that cuts people in half.
It says like the wind, not identical to it. This is my favourite for many reasons. The nostalgia for HoMM II probably chief among them.
For me it's too hard to narrow it down to just one card. That's why I chose one card from every different card (color) class: Armor: Inspiring Armor Assist: Bless Attack: Deadly Spark Block: Disorienting Block Boost: Firestarter Handicap: Traveling Curse Movement: Scamper Utility: Telekinesis
I was going to say Dissolve Armor, but then I remembered about Whirlwind Enemies, and how I think I saw an episode of Masters of the Universe end in the exact same way. So....I'll go with Zap. Quite possibly, the perfect illustration for a card.
There's a lot that I like. Swap is really amusing, so is Cowardly, and I find Skillful Dodge really adorable. On more serious notes... There's just someone I really like about this one. It looks really menacing. For a block especially. However, the winner (for now at least) is something else... I think it looks great and satisfying. I'm biased, I'll give you that, I like the concept behind it. And I just had to quote this, because I feel the exact same way. I'd probably have chosen those as my examples, too! I'd probably have added a few, like Bless, Strong Hack and probably something else I've forgotten.
Yes I agree - Strong Hack is also a very funny one, but I just wanted to list 5 and be done with it. I can list the 20/25 I had/have in mind if you guys want.