How can I tell what version of an LP is best ?

I think that if you just google "What is the best pressing of..." you should come up with a good idea. Trying to find a great copy of a record is part of the fun. Plus, there's a lot of new, re-released stuff that people have purchased and reviewed. Don't get crazy with it. It's supposed to be enjoyable, not a source of grief.

I agree, Saki. Replacing an LP record library with CDs was madness. (I did warn them in 1990 but would they listen? :D)

Not sure if you meant you had listened to 3 versions of a CD or 3 LP versions of a CD but if the latter I'm mildly surprised. I've generally found a lot of pre-owned LPs are of very marketable quality once they've been cleaned. Indeed many of them will sound better than some brand new LPs - even of the same era.
As a rule I've always preferred the pre-1980s pressings because there was less risk of digital contamination in the mastering chain as digital recorders were developmental.

Pressing quality can vary depending on the condition of the stamper, whether it was at the beginning of a run or the end of a batch. Then, factor in the poor handling by the previous owner and it can lead to an unsatisfactory outcome even if the original pressing was a top 1971 example produced with brand new stampers.
Unfortunately this means that the buying process is very much trial and error. Even if there are codes & numbers inscribed on an LP you can still end up with a load of rubbish.

Audiophile 180g/200g LPs are very much a mixed bag. With some, the mastering and production are impeccable, while others can't even get the hole properly centred with the label such that your stylus collides with the edge of the label!!! (Be alert!)
Also you should be aware that the recording content or master tape may not even be the same one that produced the cherished recording, so well loved, from that bygone age. Compare e.g. Robert Fripp's LP "Exposure" with the CD version to be treated to songs which were clearly recorded at a different session or alternative "takes". Whether this is because they couldn't find the original master tape, whether it was damaged etc I can't recall, but it happens. (The CD version is actually a bit of a curiosity for this very reason. Interesting - but I won't go into that here :D )

The encouraging thing to draw from all this is that 2nd hand LPs are not a waste of time and a high percentage of them are well worthwhile - sometimes sounding like new. A line contact stylus will often evince a sound quality which the LPs previous owner never thought possible from his "old banger", featuring vanishingly low noise levels and unimpaired dynamic peaks.
Similarly, even the most vicious and lengthy looking scratches can be completely inaudible on replay.
If you do re-enter the fray it will be a voyage of discovery but one that only you can make.
Hope this is helpful...

Very well stated by Moonglum. But I must say, that like different cartridges, tonearms, preamps/amps, speakers and cables, it's what oooos and aaaahs *YOUR* ears that will tell the story. Personally, I almost always prefer the sound of original pressings over the reissues. While there are those who latch onto the dynamics and clarity of the latter I usually find the former easier and more traditionally "tube like" on my aging ears---and frankly, more natural sounding too. So, prepare for a return to those "thrilling days of yesteryear". Happy hunting and good listening!

Agree with the above and will add that Gemm.com is a good source for prices.