What's the highest possible attack rating for a melee weapon in dark souls?
Assuming max stats and using any enchantment on any weapon. Right now I am getting 706 rating with the crystal demon's greataxe +5.
The Demon's Greataxe has the highest potential raw damage, due to nice base attack rating and S-scaling in Strength. A Crystal Demon's Greataxe +5 wielded with 99 Strength has 767 attack rating.
Note : Two-handing doesn't boost damage at 99 Strength
This weapon can be enchanted.
The best talisman is the Darkmoon Talisman. With 99 Faith, it has 240 magic adjust, resulting in 504 magic damage for Darkmoon Blade.
Starting with the Cleric class, this is SL174, long but not that hard to achieve. No ring nor equipment piece can boost this rating.
Here is the build on Mugenmonkey.
Praise the Sun !
I disagree with the previous answer. A rating closer to the actual maximum would be the 2132 AR achievable by tumblebuffing a +5 Grant with Crystal Magic Weapon using a Tin Crystallization Catalyst + RTSR + Power within's 40% (1523 is the total displayed amount, as PW's AR increase isn't shown in menu). Stats were 50 STR, 50 INT and 50 FAI, so you could expect even more. edit- I'll do some testing later today with an Enhanced Great club +5 with Darkmoon blade +3 + Darkmoon talisman and Crystal Magic weapon + Tin Crystallization Catalyst.