At the most simple level, the beer you HAVE to know...and by know, I mean understand to an explicit level...are:
- Pilsner Urquell - epitome of pilsner
- Hoegaarden Wit - epitome of Belgian witbier
- Weihenstephaner Hefewiezen - epitome of German weizen
- Little Creatures Pale Ale - epitome of American pale ale
- Coopers Extra Stout - epitome of foreign stout
...and if you have time/money/availability...
- Chimay Bleu - epitome of Belgian dark strong ale
- Rodenbach original - epitome of Flemish red sour ale...if you can handle it
- Cantillon Gueueze - epitome of Gueueze lambic...yeah, if you can handle it
- Timothy Taylor Bitter - epitome of British bitter
- Fullers ESB - epitome of extra special British bitter...see the above.
- Fullers 1845 - epitome of Old Ale
- Newcastle Brown Ale - epitome of Northern English Brown Ale...
...just to name a few.
But yeah, there's a huge number of styles to begin with, and this is just a list of 12 typical examples of each.