What are some top tier PvP weapons to design a build around?
Estoc/Rapier + Buckler/TS/SLS for the boring parry&poke gameplay. Only makes sense if you learn the parry timings and know how to use your 2hit recovery on spammers. Use the Painting Guardian Sword in your off-hand for style parries.
Washingpole, Dragonslayer Spear, Gargoyle Flamelance, Dragonslayer Swordspear are all nice options if you enjoy your range game and want to punish mistimed attacks or rolls. The Dragonslayer weapons have nice weapon arts that can catch people off-guard.
Fume Ultra Greatsword (even after the nerf), Yhorm's Great Machete, Black Knight Greataxe are the best heavy weapon choices in my opinion. Normal Greataxe is also decent but the lack of range makes you very vulnerable against the pokers.
Twin Princes Greatsword, Astora Greatsword, Farron Greatsword. Gimmick weapons that will catch most inexperienced players off-guard. You will lose to every half-decent PVP player because these weapons leave you wide open with their weapon arts/special attacks.
Black Knight Halberd. Probably the best 1on1 weapon in the game if used correctly. Very hard to nail the punish combo timings on low weight + Carthus Blood Ring users but doable. Learn how to buffer running attacks if you want to play offensively. Weapon is shit-tier in open PVP against multiple enemies.
Some weapons I haven't tested myself but I've seen people do amazing things with them:
Morne's Great Hammer, LK Long Spear (infinite poise on attack???), Demon's Fist, Immolation Tinder.
If you want to go full try-hard always use buffable weapons with Moonlight/Sunlight Blade buffs. Estoc and Washingpole are already well established in that category.