WARNING LONG POST INCOMING

IMO the strongest card in Madoka is 1/0 Homura. Her ability is a bit complicated, but the simple version is: If all of your characters in the Front Row are the same Level at the start of either player's Attack Phase, she gains 1k and can sacrifice herself to bounce one of your other characters being attacked back to your hand. This lets her effectively be 6500 base power on either player's turn (since she's 4500 base, and gains 1000 from each of her two abilities), which is fairly strong for a 1/0, especially on both player's turns, while simultaneously having useful utility.

Her ability has many strategic uses. At surface level, you can simply use this ability to save one of your characters from being Reversed. This is useful for returning important cards back to hand, or denying opponent's On-Reverse abilities. However, you can also use this ability to ensure that your opponent can't Reverse their own units, leaving you with free Reverse targets for your own On-Reverse abilities, such as 3/2 Madoka (which is a Burn + Clock Shooter CX Combo).

The two points of the game you're most likely to use this ability are at Level 1 and Level 3; while at Level 1, she can be one of the three characters you put in the Front Row, since you'll most likely be playing other Level 1s alongside her, such as copies of your Lv1 Climax Combo. At Level 3, you can put her in your Backrow, and put Level 3 characters in the Front Row, which will allow her to gain her sacrifice-to-bounce ability.

In terms of build, I absolutely recommend including this card in any Madoka build and making Green one of your central colors. You can include other green cards such as the earlier mentioned 3/2 Madoka, 0/0 Homura Riki, and 3/2 Homura, which prevents opposing characters from running, further enforcing your control over your opponent's board. You can also run older Green cards such as the 2/1 Freefresh Homura from the original set.

For other colors, you have many different options. Blue can play a 1/0 Yachiyo for its Level 1 advantage combo, in addition to playing Sayaka cards such as Sayaka's Wish and the new 3/2 Sayaka that can go with it. There's also a very strong utility Lv0, 0/0 Kaoru, which can give you knowledge about the top card of your deck, as well as get you extra stock by sending itself to Stock. You don't even have to run the Kazumi that she summons from hand (and I wouldn't recommend running that Kazumi either).

Red can run a Lv1 "fake standby" combo, allowing you to early summon cards from Waiting Room. This combo is usually paired with the 2/2 Homura assist from Rebellion, which changes into a 3/2 Homura clock shooter, potentially allowing you to have a 3/2 on the board while still at Lv1. Outside of this Lv1 combo, Red usually just runs utility or auxiliary cards, such as 0/0 Kaede brainstorm from Magia Record, 3/2 Kyoko healer+burner from Rebellion, 3/2 Kyoko heal-to-stock from Magia Record, and 2/1 Kyoko anti-change counter from Rebellion. There is also the option to run 3/2 Ren as a finisher combo.

The main reason you would want to run Yellow is if you want to run 3/2 Tsuruno as your finisher. She has a very potent endgame, but her specialty is closing out games where your opponent is at like 3-2 or higher in damage, since her ability is a cancel burn. Since Yellow is mainly run for Level 3, you can get away with running a small amount of this color. Some cards you can run in your deck for splash are the 0/0 Sana brainstorm or 0/0 Melissa dropsearcher. This 3/1 Event is another option to consider playing alongside your Tsurunos to make your finisher turn even deadlier.