I'm still very much a beginner, so I haven't got any hard and fast requirements as yet, but this is certainly worth thinking about (though I'm trying not to buy any more decks for a good long while).
One card I'm often drawn to is the Two of Swords (especially the Morgan Greer version, and the classic RWS - I also rather like the Fey Tarot one and the Wildwood "two of arrows" is pretty stunning).
The Tower seems to be a fairly important card for me. One of my favourite cards in the Green Witch Tarot is the "Wild Hunt" card which is pretty much the Tower in her system. It just looks kind of epic, plus the tower in the picture reminds me of a tower in Skyrim which kinda works well thematically (but I won't bore on about that any more!)
I don't necessarily need a GOOD Magician, but it's an easy card to get badly wrong - I really like the overall style of the Hanson-Roberts Tarot, but oh that Magician - goofy Fabio... (With apologies to any fans of this particular card!)
The "heavenly bodies" cards - star/moon/sun - are all ones I usually look at (the Pole Star from Wildwood... gorgeous!)