I haven't played many RPGs, no particularly reason just other genres appealed more but I bought The Witcher off GOG recently and I love it – it’s a really great game and it's made me think that I really should try out some of the classic RPGs.
So I have narrowed my choice down to Baldur’s Gate 2, Planescape Torment, Fallout 2 and Gothic 2 but I'm can't decide where to start. Does anyone have any reason to think any of these would appeal more to a relative newbie to role playing games?

If you liked The Witcher, go for Gothic. The first one. There are far too many sequels in your list, anyway. That's the closest game to TW of the bunch.

PS:T is the hands down best game of the list, but will be better after you've played through the BGs, because you'll be familiar with the interface, and that helps. Play BG before BG2, the story continues directly where it left off in the first one, and it's more epic that way. Fallout is better than Fallout 2, so there is no reason to skip that. It's more difficult with the Gothics, both have their strengths and weaknesses. Play both.

BG2 does have a great story, but to appreciate it fully you need to play the first game. Also BG2 can be quite a big difficulty jump since it starts off, essentially, as a continuation of the first game - so it expects you have some idea what you are doing. ie you don't start at level 1 in a nice easy area - you start around level 10 in an easy area (and then once out that it gets much harder very fast).

BG1 is an older interface and can be a little slower to get used to, but overall its good to play these things like reading a book - start with the first and work you way through.

Also BG2 is one of the undisputed top RPG games for story and depth - not least of all party depth whereby party members do have some mind of their own (most notably some won't get along with others can can come to blows over it - even killing each other)

None of the games you listed are newbie friendly.

No point starting with the second installments if you have not played the 1st (BG, Fallout, Gothic). Except for Gothic, and that is also a bit far fetched, all the others are not really start-up material and the rule set will certainly not give you a Withcer like experience. Don`t get me wrong . .they are awesome games. I would recommend Divine Divinity and Beyond Divinity as really nice games .. and then Gothic. You could also just start with Gothic .

I guess it makes sense that I should start with the first game in the series. It's a bad habit of mine to pick what seems to be the best game and start with that. But I accept that for RPGs there might be a learning curve that I'm not taking into account.
Okay so if I was to choose between Baldur's Gate, Fallout, Planescape Torment and Gothic instead of the sequels which would you suggest?

Then try first the originals.

I don't know for BG, but the first Fallout has much more, well... integral story then the second, so you have less chances to be confused by anything. I'd start with Fallout series, continue with Baldur's Gate, then Planescape: Torment and, finally, Gothic.

Of these, BG would be the least unforgiving for a new player.

I've never been a Gothic fan (just never got on well with the demo enough to go further) but you might find it more similar to the witcher in interface.

The others in your list are a very different kind of interface and RPG experience. Of the three I'd say that Baldur's Gate (with plans to move onto BG2) is the one I would go for first; but Planescape I've seen good things of. Fallout is - much older and as such not only harder, but a more rustic interface again.

If you like the witcher I have to agree with the person who recommended Gothic. It is closer to first-person (well, it's over-the-shoulder) action than the isometric BG, Fallout, etc. I have only played Gothic II, but is is probably my all-time favorite of the list (maybe of all time), and I LOVE all of them. Gothic just has an aesthetic I like, plus there isn't any level-balancing - you just can't wander into some areas until you're strong enough. It's tough, but worth it.

Well actually, while fans will of course claim otherwise, BG1 didn't quite age well. There are interesting bits, overall it's just tedium, and the story can be summarized into one sentence - BG2 will do that for you gladly.

Fallout 1, on the other hand, is the same game as the second part, only smaller and more compact - it doesn't really matter that much which you play first thou.

Oh, and you'll do the best if you just pick Planescape. Seriously, you like The Witcher and haven't played Planescape..

Pick Fallout or, even better, Fallout 2. It's classic, has great story, was my first RPG, so, i guess, is newbie friendly. It's what got me into genre, it's... just my favorite. So, i can recommend it. :)

choose what you like most
- pure fantasy, sorcerers, warriors, thieves, all the god stuff: baldurs gate
- heavy story and interesting places and 4even more interesting charakters, not so much your "typical" fantasy: planescape torment
- mad max, playing a charaktere you "invented" with all his strengths and weaknesses: fallout

you get lots of fun of either one, everyone of those is great

I just want to thank everyone for their advice.
I have decided to go with Baldur's gate. The story/atmosphere from Planescape Torment and Fallout actually sound more appealing to me but it seems that Baldur's Gate would be a good place to start and there are so many positive reviews for it so I'm hoping that I will enjoy it even if it is a bit dated.
I plan to play Planescape Torment after that and then Fallout so they should keep me going for awhile.