Which beer has tha maximum or highest alcohol content?

The strongest naturally brewed beer was 23% ABV (40 degrees proof in American). You can't really make anything stronger than that (including spirits) without a distillation process. There are people in Scotland, Holland and Germany who use "freeze" distilling to make stronger and stronger beers because being the record holder means you sell well (everyone wants to say they've tried the strongest).

I tried Tactical Nuclear Penguin (32% ABV, 56 degrees proof) at a beer festival and it wasn't all that pleasant. I understand things have moved on, though. The German brewery (whose name I can't remember... something like Schorssbrau) made a beer that was 60% ABV (105 degrees proof) and the Scottish company (Brewmeister, up in Aberdeen) made a 65% ABV beer (114 degrees proof). They sell it in 330ml bottles and one bottle is roughly equivalent to 90% of a bottle of Southern Comfort.