List your favourite GoG that you own. Make a top 5, 10, whatever.. list. I am actually using this kind of as a reference to what I should buy next...

Hmm, Might have to make a top 5
Beyond Good & Evil
Cannon Fodder
Prince Of Persia: Sands Of Time
Freespace 2
In Cold Blood

I only own Beyond Good & Evil on your list... was thinking about buying Cannon Fodder... of course I never played Prince Of Persia: Sands Of Time. SO MANY CHOICES!

Septerra. Hands down.
The first game I've actually found myself wanting to play for more than 10 mins at a go. Including all the current releases out there and not just the old games on GOG.

How does it compare combat wise to something like Final Fantasy 7? Thye combat in that game really turned me off, it was just too damned boring and considering that I completed the original Pool Of Radiance with its legendary "Surrounded by 200 orks that each take 10 seconds to realise the can't move or attack" battles is saying something

Two words.. Cannon Fodder

How does it compare combat wise to something like Final Fantasy 7?
I'm not an FF fan, so to be honest, I wouldn't know. Although the combat isn't exactly quick (at least in the beginning), you do too little damage I think even when charged up. But I'm still enjoying it >_>

Fallout and Fallout 2.

Kingdom: The Far Reaches

While I've enjoyed just about all the ones I've purchased. The most recent purchase of "Beyond Good & Evil" currently sits at the top, even if I nearly tore my hair out on the final boss fight, it's really a few steps above the rest.
Others mentions go to Freespace-2, Timeshock Pinball and Fallout 1.
I'll include Sacrifice, I already have it twice on CD, so I don't quite need a gog version just yet.

I don't want to talk about favorites - the other games will just be envious. But it's pretty clear that the purchase I've had the most fun with so far is Capitalism Plus. It's a fantastic game.

Freespace and Fallout.

The Guild Gold for now..
"You have gained a new worker!"

Gothic 1 & 2 (Fantastic open ended action/RPG, quite frustrating and tedious level up system though).
Freespace 1 & 2 (Best space shooters since Wing commander).
Toca 3 (Lots of cars and race modes, challenging and fun).
Duke Nukem 3D (Great old school shooter, although it still revolves around the find the key to open the door routine, the level design is far better than most for it's time, plus the HD pack makes it look and play so much better).
Any of the adventure games ( Point & Click games have always been my favourites, especially the older ones (late 80's - mid 90's).
I could go on, but i would end up listing half the game catalogue.
I guess it depends on what you like. There are quite a few strategy & RPG games here which i know are fantastic but i am utter crap at them.

My top 5 would be the following:
Fallout 2
Prince of Persia
Cannon Fodder
Far Cry
Freespace
If you've not played Fallout, then ElPixelIlustre is right in saying you should get both and treat them as one game, its well worth it.