It really depends on you. When I was younger and more casual I found Trickster the most helpful. But now Swordmaster suits me best. However Royalguard, in the hands of someone who can master it (so not me lol) can be the most devastating style.
Royalguard for me definitely. Took a while to master it, but once you have, then you're basically invincible. On the other hand, I'm devoted to Trickster as it provides more flexibility like DMC2's combat system did.
1. Swordmaster - It got me many out of a tough spot and a whole bunch of Style points.
2. Trickster - Lv 3 is where it's at.
3. Royal Guard - I mainly on use this for certain bosses: Hell Vanguard, Agni & Rudra, Bewoulf, Nevan, and Vergil. Some lesser demons too.
4. Quicksilver - It spices up the variety, but is a glorified bullet time button. Useful against DMD Vergil 2 & 3.
5. Gunslinger - Only fun on difficulties below DMD. Otherwise, it is too impractical and tedious to use.
6. Doppleganger - I've only used this about 3 or so times. Too gimmicky.
Swordmaster was my go-to for the longest until I started investing in Gunslinger. Trickster was for when I REALLY needed it. Royalguard and Quicksilver were my least used abilities, with Doppelganger never being touched.