It might not be such a bad idea to join the current GW#1 right now. It's a mature game that's starting to wind down, the mega airlines have mostly maxed out their bases and are too busy doing fleet renewals to focus on growth. And there are quite a few airports with no airlines based there including:
Any of those would be a good place for a beginner. Especially Muscat. There are no airlines headquartered in Oman so you wouldn't have to worry about a gorilla opening a base there. The Arabian Peninsula is a bit crowded otherwise but there is good demand to be had from MCT, and with the 737MAX and A320NEO now available, there are lots of A320CEO and 737NG on the used market, both of which will cover most of Europe, Asia, and Africa from Oman. There are also plenty of E-jets and CRJs available on the UM, for routes to small airports with no hometown airlines and therefore no competition. There are even some fairly young 737-400 available.
Everyone is afraid of joining games - especially long ones - late in the game. It really isn't as hard as everyone thinks it is, and I would argue that it's actually easier because there are fewer players than at the beginning of a new game (only 204 now vs. 700 at start), and fewer still who are dependent on the UM for their entire fleet. At the beginning of a game nobody can afford to order new planes so everyone is clamoring for whatever they can find in the desert. At the end of the game, it's mostly just startups shopping on the UM. The old guard are getting their planes brand new from Seattle and Toulouse.
And starting at the end of a game is a good learning experience because while you do have competition from the other end of the route, you also have plenty of uncontested demand to small airports. Just don't be afraid to put one 737 on a route with 1000+ demand and 1500+ supply. You won't fill up your 737 but you will get butts in the seats. And with the game nearing the winding-down stage, a lot of airlines are in "maintenance mode" right now - just logging in to swap out planes for D-checks or to park a 25-year-old clunker after their shiny new MAX arrives.
One more good reason to join a mature game is that a new long game world is about to open, so everyone's attention will be there in a couple of weeks. When the new GW2 opens there will likely be a further drop in the number of players in GW1, as players with small struggling airlines drop out in favor of restarting in 1960. Even better for startups in the mature game.
As to the other two long worlds, GW4 is just in 1998, it's about to hit the 2001 downturn so I wouldn't recommend starting there right now. Maybe towards the end of February after some of the bigger players go bankrupt and global demand recovers. And GW3 has less than a month to go, that would be a waste of credits to join the game now. At ~4 months, GW1 is in my opinion the best place to get your feet wet. Go for it!