Kirschbaum makes good poly strings.
Gosen is widely known for good, cheap strings. Mainly synthetics, but Gosen Ice (poly) is really good too from what I heard.
Technifibre makes excellent strings. I believe they're the ones who actually manufacture NXT.
It'd probably be easier if you specified which strings you were looking at.
[K]Six.One Tour (3) 367.5 grams 31.7 cm balance.
Mains: Babolat/Wilson Natural Gut @ 23 kg // Crosses: Luxilon Alu Power Rough @ 21.5 kg