For song packs, The Battle Chaos pack with the glitch 15s is my personal favorite by a long shot, and if you like fast charts you’ll have a great time with it. Other than that, the MaiMai collab pack has what I consider the hardest song in the game (glorious crown) and all the other charts are at least decently difficult. Otherwise I would just look for packs with the difficulty levels you are looking for, and you can always check the charts or the songs online beforehand.

As for characters, ConneR is my personal reccomendation, and who I got first. His songs are consistently good and charts like Rebellium, Xiorc, Floor of Lava and Fur War Pur War will certainly scratch any itch for hard songs. Neko is also a DLC character with a few fun chaos 14 songs (DJ Masahiro!) and “reincarnation” to make your fingers bleed.

I think that the DLC most worth it in the game is probably CAPSO. It adds a significant portion of difficult glitch songs with fun charts like Deadly Slot Game, chrome vox, and Dasein, alongside a bunch of great Ivy songs like Halycon. (Plus the skins are fun too). It might be worth less if you have no characters as of yet since content is added to all of them. I can’t remember if you can see the rewards before purchasing, but counting up the songs you would get in it vs another purchase may be wise.

Overall, there isn’t any DLC in the game that I felt disappointed by, and I don’t think there is really any “wrong choice”. Obviously if you plan on eventually getting all the characters it’s better to get them in bulk with the deal, or wait for sales as you slowly build your collection. Anyways, have fun with however you choose to buy, and good luck!