There are three types of character which I think are good.
First, I think a character is good if they are likable and/or believable and/or relatable, and they don't even need to be particularly complex or developed. For example, Satou in Ajin is mostly two-dimensional, but I think he is a good character simply because he is everything I envision when I think of the word "evil". On the flipside, someone like Hinazuki Kayo and Katagiri Airi from Boku Dake ga Inai Machi, Aisaka Taiga and Takasu Ryuuji from Toradora, Kino and Sakura from Kino no Tabi, Nello and Alois from Flanders no Inu etc. are good characters because they are all likable, believable and relatable, with Sakura being a great example of a good character which isn't particularly developed or complex--given that she was only in the last episode of the series--to the point where I felt emotionally connected to the character, whereas Katagiri (at least in her manga incarnation) has a character which is complete genius, serving not only as a vitally important plot device to push the story ahead (as well as the romantic focus--which triggered literally everyone), but also as an amalgamation of all of the main and major supporting characters' personalities barring Yashiro--for whom she was the utter antithesis--and as such being almost like the web at the centre of the cast and almost acting like a link between the past and present in the story without even going into the past. Hell, the mangaka (as well as the adaptation's director) did a pretty good job at making the personality parallels to characters like Kayo as blatant as possible. The majority of the major or cast of FMAB (Elric brothers and the major supporting roles) comes under this group.
Alternatively, another way you can design good characters is when you create two which are "joined at the hip" so to speak and are at their strongest when they play off of to form a great character dynamic. The most recent example I can think of (please note I'm not up to date with this season in any way, shape or form, so there may be a more recent one) is Chigusa Kasumi and Chigusa Asuha from Qualidea Code--who were two of the best characters on last season despite how spectacularly average the show was as a whole for precisely this reason. The very sarcastic tone the two characters shared combined with the subtly funny banter, spats and other interactions that went on between them, such as in the sidecar scene where the vehicle breaks apart and Asuha leaves Kasumi in her dust as he calls out half-heartedly towards her. Another example (and one which will probably make some people roll their eyes for me bringing up) is with Sora and Shiro in No Game No Life, where everything that makes the characters entertaining to watch comes from their well-designed dynamic.
Finally, a character can be good just for having a gimmick which they execute really, really well, such as Sakamoto or Tanaka. I'd elaborate further but honestly these should be super self-explanatory and as such feels redundant to expand on it, as they are usually pretty obvious when you see them.