Signum Pro Poly Plasma....
I know, they're rather "old school" but work well for spin, durability (as far as 18g goes) and tension maintenance.
I actually use 17g now in my X's with VS Team in M's. I think this combo gives me way more spin & control.
Blade 98 18x20 x3 - Babolat VS Team 17g @ 50lbs M's/Signum Pro Poly Plasma 17g @ 46lbs X's (5% pre stretch)-USRSA Certified Stringer-Babolat Star 5