The best year to get is the one that is "in good condition". Since you posted in the mk3 section, I'll assume that you are looking for a 1996-1999 model TDI. Those cars are all the same trim level, most don't have ABS, all have front airbags, all were 5 speed, there is no really special thing that I think separates one year from another. Just keep your eyes out for a good deal and if it's in good shape, no rust, go for it.
There is a checklist at the FAQ at the top of the forum that has more tips. If you want a newer car, the later generation, the 1998-2006 New beetle and 1999.5-2005 Jetta and Golf all have running changes. Later cars tend to be better equipped, are slightly different, etc., check out the buying FAQ in the forum and search ebay and autotrader and you'll get a good idea of market prices, what to look for.
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