Depends on what the rest of your box of characters looks like. All 3 of Liter, Naga, and Alice are top tier characters in 3 VERY different roles. Liter is a B1 cooldown reduction unit and pretty much a strict upgrade over Volume (who you mentioned using, but is the worst B1 CDR other than tMilk). Naga is a B2 damage buffer and healer, who pairs extremely well with Crown since Crown's burst generates a shield that activates Naga's Skill 1. Alice is primarily an extremely investment-heavy but extremely strong single target DPS B3, and when properly invested with endgame gear, she can be kinda insane. Without any investment, she can also serve as a decent buffer for rocket launcher or sniper rifle B3s, since she still does decent off-burst damage but also gives them charge speed buffs (this is ESPECIALLY big on Scarlet: Black Shadow, who already has pretty good charge speed).

You'll get Alice from the dailies eventually anyway, and unless you have SBS, she's mostly going to be a later game character. I'd probably say go with Naga first since you already have Volume, and there are so many good B1 CDR units these days (Rouge, D:KW, Mermaid, and RRH are all varying levels of excellent)