It really depends on what type of battler you are. For example, you may wish to trade-off Attack/Magic for an ability to increase the
Chain quicker which can be very helpful in certain battles.
However, the weapons with the highest base stats are:
Arcus Chronica which has 140
ATK and 200
MAG and costs $150,000 (although less with fragment ability). You need
Adamantite to buy this weapon off Chocolina (after end game) which you can get from Chocobo Racing in
Serendipity or by defeating Long Gui in the
Archylite Steppe and you must use two other weapons which you can get from defeating two of the quest bosses (Ochu and Immortal) in the same place under different weather conditions. You just speak to the guy by the weather machine to activate these quests.
In Paradisum which has 200
ATK and 140
MAG and costs $150,000 and can be obtained in the same manner as Serahs items.
Lastly, you can also buy the
Chaos Crystal from Serendipity and give this to Hope in Academia 4XX AF and he will give you either
OdinBolt(Serah) and you can buy whichever one you don't get from Serendipity (for 10,000 coins) or from Chocolina. These can have up to 220
ATK/MAG but it is dependant on the amount of fragments you have collected with 160 equalling the max 220 on the weapons.