User IntercardfaceNo company makes sufficiently good playing cards. Some designs come closer to the ideal, but none check all the boxes. It shouldn't be so hard, but apparently it is.

The four suits should be in
four clearly different colors. More generally, in a 2-axis deck of suits × ranks, the
n suits should be in
n colors, and the color distribution should solve the
Christmas tree problem.
We can certainly argue if spades or clubs should be green, or whether the diamons should be blue or orange. It's hard. If you pick orange, like in the Skat Turnierbild (left image above), your orange is either too similar to the red hearts, or too light, failing to contrast from the white background. If you pick blue, as in the Copag 4-color deck (right picture above, with the four aces), you're prone to choosing too similar a hue for the blue diamonds and green clubs.
Nobody wants a 1-color 4-suit deck with black hearts and diamonds. Why should anybody want a 2-color 4-suit deck? This is such a basic UI principle: Make different things look different.

Well, some solitaire games have rules for packing
red suits only on
black suits, hmm ... the answer must then be to still have 4 colors, but choose more similar colors for hearts and diamonds, e.g., red and orange, and two darker colors for spades and clubs, e.g., black and a moderately dark green.
Above picture is my
unpublished Turnierbild Dark, a 4-color dark-mode mod of a card deck for
PySolFC. I wanted a 4-color deck and chose the Skat Turnierbild colors, mainly because I'm used to them, and also because orange diamonds and red hearts are reasonably close. But the spades and hearts look too similar in lightness for me. With this deck, it's harder than necessary to play games with alternating red-black packing. It's better for games that pack in suit or regardless of suit.
Hard to say how to improve it, given that I also want the dark background. Maybe the background should have one hue for clubs and spades, and a different hue for hearts and diamonds? We're getting really experimental here! But it's UI with Simon, what else do you wish for.
Index size. The index is what's in the corner: A number/letter and one suit symbol. Regular index (left picture) is too small, even while the card is in your hand. Jumbo index (right picture) is much better. Maybe it's a tad
too big already, and the middle is ideal? Hard to tell. We want to fan it in hand and also see it clearly on the table.
If I'd have to choose right now, I'd pick that middle size. It's sometimes called Blackjack index, but that term is hard to websearch.

This is Magnum index, offering a logical extreme to the size problem. The entire card is nothing but indices. In hand, you see the good Blackjack-sized index. On the table, you see the big centerpiece.
It's an interesting idea, I've never tried it, and I'm neutral on it. But I've never seen it with four-corner indices (i.e., all four corners feature a Blackjack-sized index), which brings us to ...
Four indices. American cards put indices in two corners only. Does no American ever fan cards in hand in the wrong direction? European cards always have indices in all four corners. You can fan them both ways. I'd estimate that 20 percent of Europeans fan cards the uncommon way, i.e., the direction in which they wouldn't see indices on American cards.
Always put four indices! This is really easy to accomplish and everybody should always get it right. One counterargument to four indices is clutter, but the solution to clutter is a good box, not to restrict your fanning-in-hand direction. Which brings us to ...
The box. Surround the interior (everything that is not a corner index) with a box. Here, the eight of spades has no box, but the ace of hearts has this box. Boxes are common for court cards in American-style decks, but you should do it on spot cards, too. Do it everywhere!
Some Bridge decks have a yellowish hue in the box. That makes it even better to distinguish the in-box design from the out-of-box corner indices. I like it.
The box is the correct solution to clutter. Then you can have big indices in all four corners and it will still look good.
Card width: Practically always you want narrow cards; a common size is Bridge-sized. Poker-sized cards are wider and harder to fan.
The community cards in Texas Hold'em might look better on the table with Poker-sized cards, but that alone is a weak reason for such an unwieldy card format. If you want readability on the table, you want clearly readable, big, box-separated indices. Card width is secondary, but if you get big indices with your wide cards, that may be a good reason. For Texas Hold'em, consider the 4-color Copag, see the image of the four aces at top of this post.
The game of Tarot makes you fan 18 cards in your hand, and, as a result, Tarot cards are even taller than Bridge-sized; they're about 1:2. Not sure about that size for general use.

This is one of the best designs of physical playing cards: Desjgn Classic Circle.
- Four colors, even though clubs and diamonds are slightly similar,
- a box with light yellow background,
- Jumbo index, nice and big, maybe 5 % too big,
- Bridge-sized, not too wide, not too narrow.
Downsides:
- Cards have only 2 indices, not 4 indices in all 4 corners.
- The deck is surprisingly expensive.
-- Simon