Collection suggestions
Rainman,
Like may of the posters have said, there is no such thing as a "definitive" essential game list. However, the way I see it, you have three basic collection options:
1) Collect the titles you personally want, period. If you remember it, like it, and can afford it, go for it.
2) Collect a genre you like: fighters, shooters, puzzlers, drivers, sports, golf, imports, vectors, cocktails (etc.) or a particular company or system lineup (Taito, Neo Geo, etc.).
3) Collect things that make you go "ooooh". Things that catch your eye, things you can fix, things you can show off, things you can gift, or things you can sell. Whatever floats your boat.
If none of these options appeal to you, then consider building or buying a dedicated MAME cabinet.