The best pocket watch to start with is not great granddad's railroad pocket watch. STOP TOUCHING GRANDDAD'S WATCH.
We don't actually advocate for you to start with antique pocket watches at all; because of their age and the fact that many people have had their sticky hands in them, they are very often an exercise in frustration for a new watch enthusiast. Yes, there are a lot of them out there, but they are also old, and new-old-stock parts are getting harder to come by. And you will break something that will need to be replaced. With that being said, if you feel you absolutely must start with an antique, get a 16s or 18s Elgin. They are big, sturdy, and there are more of them than you can shake a stick at if you're the stick-shaking type.
MANUAL WIND WATCHES
Let's start again. The best watch to start with is one you have absolutely no emotional attachment to! Get an ST3600 off of Ebay. The ST3600 is a clone of the ETA/Unitas 6497. It is very reliable, times well, and most important it is large enough to not cause distress. You can obtain one from our good friends in China for approximately $50. At that price, get two, so that when your yoke spring goes flying off into the ether, you have a backup.
Grab yourself a Seiko 5 Sport for about $100 off Ebay or Amazon. While you're there, grab an extra movement; the NH35 or NH36 is a very reasonably priced automatic movement.