1701 was probably the worst anno because of annoying blurred and bloomed graphics. Everything looked like Oblivion.
Other than that all the games are pretty similar. I've never played 1503, but all the 1602, 1404 and 2070 are good to start with. It's up to you as except minor gameplay changes they just have different art-style. It's the series that hasn't devolved with the time, it stays true to the original. Get 1602 if you have weak hardware or prefer 2D graphic, get 1404 if you want beautiful 3D graphic and some different factions and get 2070 if you prefer science-fiction setting to... well, as the names of the games suggest.
I can't really say which is the best, I love all the 1602, 1404 and 2070, though myself I spent a damn lot of time in 1602, less in 1404 and unfortunately the demo of 2070 had disappointing performance on my laptop, so I had to give up the purchase and fall back to 1602/1404.
Just don't do 1701 (albeit it's pretty much the same good game, just the FUCKING BLOOM AND CONTRAST shit fucks it up)