Depends on your build and levels, but the best scaling melee weapons are going to be multi-stat weapons. For example, something like Lorian's Greatsword has 4 stat scalings, which is a negative for your general SL130 builds, but as you level you can bring up auxillary stats like Int and Fth, and at 40/40/40/40 or even 40/40/30/30 you're going to get some of the best unbuffed ARs in the game, and buffing with the WA is going to knock that up another notch. Other weapons that scale high into the NG+ cycles are the Black Knight weapons, particularly the BK Greataxe, the Greatsword of Judgement, Profaned Greatsword, Ledos Gresthammer. The absolute best ARs you can reach are going to be buff builds. IIRC, the Lothric Knight GS buffed with Lightning Blade or DMB is the highest AR that's easily achievable and useable without min-maxing with Red Tearstone Ring, Morion Blade etc. Refined weapons also perform really well with buffs, like The Greatsword and the Exile Greatsword.
The Dragonslayer Greataxe does have one of the best ARs in the ultra high levels, though, so if its damage is dropping off for you, it might be good time to start stacking buffs like Sacred Oath, Lightning Blade, DMB (if you can get it), Oath of Sunlight, Power Within, etc.