Tips koikoi

Card value

A list of all cards from least to most useful. A card's usefulness depends a lot on the situation, but these are good general rules of thumb.

Group A - Sake Cup and Brights (except November/willow):

The Sake Cup card. Take it if you can. If Viewing yaku are enabled, it is easily the most valuable card in the game, because it’s easy to make a yaku with it, and it also counts as an extra chaff card.
The Curtain and Moon cards. They can make valuable yaku with both the Sake Cup and other Bright cards, so they have plenty of options to give you points.
The Crane and Phoenix cards. Just like the cards in group B, you need two other cards to make a valuable yaku with these. However, you don't need two specific cards, so you have more options.

Group B - Ribbons and Animals that can make special yaku:

The Poetry Ribbons (with text on them) and Blue Ribbons. These form valuable yaku that can combine with each other and with the regular Ribbons yaku.
The Boar, Deer and Butterflies cards. They form a valuable yaku that can easily combine with the Animals yaku. There are slightly fewer Animal cards than Ribbons in the deck, and there isn't another Animals yaku to combine with, making these cards slightly less valuable than the special Ribbons.

Group C - Others:

Plain Ribbon cards. Relatively plentiful, and can combine with two other Ribbons yaku.
Remaining Animal cards. There are slightly fewer Animal cards than Ribbon cards, and they can combine with only one other Animal yaku, so they are usually a little less valuable than Ribbons.
Chaff cards. They are not very valuable in most cases, because they give few points and don't combine with other yaku. However, they’re very useful once you have enough for a chaff yaku, especially when playing with Soft Koi-Koi rules. They are the most plentiful group, so it’s easy to earn more points with them. Unless your opponent is close to making a yaku, it’s often a safe bet to call “koi-koi.”
The Rainman card. The least valuable of all the Bright cards, and arguably the least useful of all cards. You need at least three other cards to earn points with it, and it only gives you one or two extra points. But can still be useful to capture if you or your opponent is likely to get three other Brights.

Cards in your hands are safer than on the field. Focus on getting strong cards from the field first, unless a card you want to capture from your hand is much more useful to you.
Keep a close eye on cards that can make a special yaku. These can be dangerous, unless you’ve already captured one of its yaku set. It’s often worth capturing one card of a yaku (e.g. taking a single blue ribbon), even if just to prevent your opponent from making a yaku with it later.
If your opponent is close to making a yaku, it’s often a good idea to capture cards from the same month as the one they need. For example, if your opponent has the Boar and Butterflies cards, capturing Maple (October) cards from the field make it less likely that your opponent can finish their yaku with a card from their hand, or with a lucky draw from the deck. (The Month and Yaku lists up top allow you to check which cards you need to capture to do this.)
If you can make a match with cards from a month whose two other cards are already captured, those remaining two cards are safe. It’s impossible for your opponent to take them. Focus on capturing other cards first, if you can.

If one of those safe cards allows you to finish or expand a yaku, that gives you the power to end the round at any turn. If your opponent isn’t close to making a yaku, that allows you to be more reckless until your opponent catches up.

If you didn’t start the round (i.e. if your opponent has fewer cards in their hand than you whenever it is your turn), then it is impossible for your opponent to capture the last card in your hand. This means that if you have a card in your hand that they need for a yaku, you can guarantee that they won’t capture it by saving it for your last turn.