Kingdom Hearts BBS is, on par with KH1, the best KH game ever made.
The battle system is the best one in the series so far. Not only does the command list add a whole new layer of strategy to the series, if you also take in consideration the form system as well, then you've got the most complex system ever in a KH game.
The story structure is also pretty nifty. By splitting the entire game's story into three, very different, parts, Nomura has created a wholesome story for each character, highlighting the true importance that each of the three Keyblade Masters have in the overall KH story arc. Also, the final chapter is a must if you want to comprehend some very important events of previous KH games.
In terms of mini-games, KH BBS also has the highest amount of them, which are also quite varied. With the Board mini-game, racing mini-game, volleyball like mini-game, ice cream rhythmic mini-game, there's enough to keep you entertained for long hours (and if you're going for full completition, then you'll want to finish every single one).
All in all, KH BBS is an amazing KH game and I throughly recommend it 
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"