This one is a beast. The grand daddy of 18xx. Sure, it was actually the second 18xx released, but let's be honest: if 1829 was the only 18xx that Tresham had released, it would likely have remained the only 18xx title.
1830 is an ingenious design, given that it had very little in the way of predecessors to build from. The stock market is cruel and unforgiving, the map has pitfalls that you must work around, and the game itself does absolutely nothing to hold your hand.
It's a very good game, and a monumental achievement in game design. If other 18xx titles are better (and some undoubtedly are), it is only because they stood on the shoulders of this giant.