Best Chess GUI

cute chess is the best

What makes something the best chess gui?  What is the criteria?  What are the most important features?

• Playing against an engine?
• Game explorer?
• Book openings?
• Searching positions?
• Game analysis?
• Using the latest trends in UI design?

It depends on individual basis.
I was using lots of UIs from early 90s till nowdays...

From my point of view, quality and stability is before number of options provided. Combined with easy and clear interface, the one should not get tired or even fed up after just 30 minutes of usage.
Next, it has to work with pgn database files. I mean here, that user can modify games, save games back to pgn, overwrite...All that with basic search/filter functions.

Here, I would set ChessKing on first place, even this is commercial GUI due its quality, even with minimal options compared to competition, it's green based theme pefectly fits to the eyes when there is long time using of it. Than also Tarrasch, but Tarrasch does not have search options in database which makes it difficult to use with larger bases. ChessKing works fine with pgn databases and it has nice and fully functional database window.

SCID vs PC is high quality UI, but  bit complicated (for example deleting single game from database) and it does not handle directly (read only mode) with pgn databases (drawback!).

Arena GUI also good, customisable, but has a bugs (some time game is automatically played to last move when using navigating arrows, sometime database games are not deleted from file, many times it does not read properly initial positions from some of pgn games - FEN...)

ChessBase, long time ago FRITZ6 or FRITZ8 or similar interfaces, I assumed as top. Now  days i do not use it, it's getting too complicated.

For playing against engine, again ChessKing and Shredder (i tried only android app) provide best experience. Long time ago was also ChessMaster.
Everything else looks very poor when it comes to the reduced engine strenght choosen or simulation of average human player moves.