Older ones are a little harder to get into than world, but, that being said, I started with 3 Ultimate. They're a bit of a different experience. The old games are pretty notorious for not holding your hand in the slightest and expecting you to figure things out on your own, while world does its damnedest to make sure you know everything by shoving a ton of tutorials in your face. World combat is a bit more dynamic in world, with terrain being something you have to take into account, while in older games, most areas are you, a flat plane, maybe some walls, and the monster, at least until 4 and Generations.
A good jumping off point in my opinion if you want the old game experience is 4 Ultimate on 3DS, it's still got a surprisingly lively community around it, and gives you a bit more tutorial than older games without world's excess of them. Generations Ultimate is a bit of a fanservice game, from my point of view, mostly appealing to long-term fans of the series with a lot of old returning monsters and old towns from older games. World was designed to pull in more players by making the games more accessible.
But yes, free content like new(returning) monsters and crossover events were added to world later, and the "Ultimate" version is DLC instead of an entire second game you have to buy. Most of the controversy in world was around it being a lot easier because of things like the ability to move while healing, and act out of animations that used to lock you in place. Really, the big problem with World right now, for me at least, is just having much shittier weapon designs than in older games and a slightly lower than usual monster count.