As I said: None. Recommend me where to start cause April 2014 is fucking far!
Okay, I think the best way to get started with this is BG2. It is not only one of the most polished, rounded and extensive of the bunch, but it's also the most accessible. PS:T is about on par with BG2 in terms of quality, but is less accessible, so BG2 will make a better introduction to the Infinity Engine. But if you're really sick of Forgotten Realms and fantasy worlds, this is might be a good game to start with. As was mentioned both by me and Jibbajabbawockky, the start will be a little hard, the battles might seem confusing and both games sort of throw you into the pool, but you should just force your way through it - Once you get into it, you won't regret it. I will recommend familiarizing yourself with some of the very basics of AD&D before playing (know what needs to be high and what is low, because that is not obvious, and know what THAC0 means, etc.)
Now, if you really liked the battle system in those two games, I would recommend you give the Icewind Dale series a try, if you didn't I would skip it altogether.
I would also skip BG1 for now, the game did not age that good (not necessarily in term of UI, which can easily be fixed with mods, but in design sensibilities. Companion interactions are seriously lacking compared to BG2 for example.) It's not a bad game by any stretch though, so after you're done with the rest of the Infinity family you would probably want to give it a go.
Before that, though, Fallout 2 is an amazing game that should also be played. It is quite different in many regards (setting, battle system, engine), but it's still share much with the IE games. Many will tell you Fallout 1 is better - Do not believe them. They are wrong. Play Fallout 1 somewhere down the road, but Fallout 2 is a superior game. But before getting to that, you still have even more great games like Arcanum (as flaws as it is) to play!
So, in short, my recommended order:
BG2 -> PS:T -> either Fallout 2 or IWD -> Fallout 1 or IWD 2 -> BG1 -> Arcanum.