What are some really good legendary primaries for PvP since I love to run Icebreaker
High impact Pulse Rifles like The Messenger, followed by mid-impact Hand Cannons like Jewel of Osiris/Red Hand IX (the high impact ones like Timur's Lash and Ill Will are not as effective in my experience, too slow). Depending on the map and your play style, a high impact Scout like The Scholar or Ghaelon's Demise (3 headshot kill) are not bad, but a lot less versatile. You'd need a shotgun for close quarters and since you're running sniper, Scouts are a bad complement. Auto Rifles are universally bad right now, don't even bother.
That said, PvP is where Primaries deserve the exotic slot. The gap between Exotic and Legendary primaries is far higher than the gap between Exotic and Legendary secondaries. Meaning, there are plenty of legendary snipers as good (and a lot better, in fact) than Ice Breaker, whereas no legendary primary can compete with The Last Word or Thorn.
Ice Breaker is high impact and has very good aim assist. It suffers, however, from extremely strong recoil so it's impossible to land a follow-up shot if you miss a headshot. LDR 5001 and Her Benevolence have the same impact/RoF while boasting FAR better stability. Her Benevolence has the additional benefits of actually having even higher aim assistance than Iron Banner and also being re rollable (you can get better sights, Hidden Hand, Unflinching and maybe an additional boost to stability or magazine size).
Having re generating Sniper ammo is super nice, but the tradeoffs are too many. Ultimately, I'd suggest you try out top-tier legendary Snipers instead of playing with legendary primaries for the most part.