That depends on whether you are looking for 12, 24, 36, or 48 volt alternators. I am only going to speak to 12 and 24 volt as they are the most popular.
For 12 volt alternators the highest output produced by Delco Remy is the 300 Amp 40SI. It comes in a Pad Mount part #8600298, J-180 short hinge #8600299 and J-180 Long hinge #8600300. The highest 12 volt produced by Prestolite Leece-Neville is the J-180 mount 4890JB and Pad Mount 4962PA both of which put out 320 amps.
There are many more to choose from but I decided to highlight these few select ones. The only thing to keep in mind when choosing that ‘High-Output’ alternator:
Higher Output = More $$$.