Ok so, The Bitcher series is obviously a KODEX fan favourite, suprising, givin that it's pretty popamole 3rd person RPG with some C&C sprinkled in..
So which one is the best one, 1 or 2?
They're different, first Witcher has a sort of rhytmic combat system (you choose weapon styles depending on what you're facing and click the mouse button in the appropriate moment so Geralt can chain combos) while Witcher 2 is your average modern action RPG with rolling. Neither system works that well and as a consequence combat definitely isn't one of the series strengths but I also didn't find either to be offensively bad so take that for what it's worth.
The first one is more about Geralt's personal quest and his profession (monster slayer) while in the sequel he's right in the thick of court drama, big scale wars and conflicts (there's some of that in the first one as well but to a far lesser degree). You can choose to remain neutral in the first one (which feels more suited to a witcher) while you have to choose a side in the sequel.
The first one has a much better alchemy system, itemization (it's limited but that's a plus in this case) and somewhat better quests on the whole but is much less polished in terms of voice acting, writing and dialogue.
The first one has more of a slavic, folkish charm/feel while the sequel is more "Americanized" but I feel people exaggerate the degree to which that is the case, it still looks visually distinct to me compared to similar themed games on the market and I really liked the attention to detail for some of the major characters (in terms of looks and attire).
The first one also felt quite longer to me but I don't have the exact numbers.
Forgot to add, the first Witcher has beautiful art/paintings to illustrate the consequences of some major choices you make in the game while the sequel has shitty flash comics instead (added by enhanced edition IIRC).
and is 3 gon be the best 1?
I think it will have the best combat gameplay in the series (Witcher 1 and 2 didn't really set the bar high in that regard), have no idea about other things. One thing I'm always afraid of when games try to go huge world, sandbox approach is that the hand-crafted feel gets lost nearly completely and it all boils down to endless repetition, recycled content and simple quests.
I'm always for quality over quantity so CD Projekt boasting about the game's size has an opposite effect on me.
Can I make sense of the "story" if I play number 3 first?
Don't see why not, you probably won't get some references to previous games or the books but that's hardly a big deal.