I'm new, about to try one of the free games, any advice on if any are good and which is best?

Beneath a Steel Sky is the best.

My first GOG purchase was Septerra Core and I beat it like it owed me money :)

I'm impressed. I remember that game took me months to plow through!

Apparently, my first GOG was Gothic 1 and 2 (back in April 2009). Have barely played any of them - whereas my Direct2Drive version of Gothic 3 for 3,95€ has by far been my most played PC game for months - but I really like having those meaty RPGs to look forward to for some rainy summer when there's nothing else to play...
Edit: Just for fun, I made a simple chart of all my purchases between April 2009 and May 2010. Turns out I bought 18 (!) games in Sept 09 due to two big promos that month (plus a few desirable new releases as well). That's scientific PROOF right there that seemingly insane promos are often a good deal for the publishers/shops in question...

My first purchases were Fallout, Fallout 2, Ground Control 2, and Earth 2160. That was sometime this past February.

Homm 3 during the Christmas sale.
People were right it's fantastic.

Screamer
I got it in order to participate to the first GoG Screamer contest.
Too bad screamer 2 didn't made it here tho... I would have loved to participate to a Screamer 2 contest too... :)

It took me a while to get through it the first time too, but that was six years ago and I can't play it on my "top o' the line" computer. When I found it here it was an instant purchase followed by a 32 hour play fest. It's one of those few games that you want to play over and over with different character line ups to see what you can get away with.