As a longtim player of these games, here are my honest thoughts about them.
Age of Empires - Not worth your time, unless the new remastered version is REALLY well done. Though the first game is not really bad, it is dated. You can't make buildings que multiple units to build at a time, farms had to be manually rebuilt every so often, and the entire game is just full of micromanagement and busywork.
Age of Empires 2 - Though superficially similar to the original, this is arguably the best game in the series, especially the HD version. Most of the archaic BS from the original is cleared out, leaving you with a solid package. The game benefits from having a lot of stuff added in expansions over the years, the latest of which only came out in 2016, so you have have a huge number of factions, scenarios, and so on to play through. The AI is a bit flawed at points, and the differences between the fctions aren't as drastic as in some other RTS games, but this is a good package.
Age of Mythology - Honestly my favorite entry in the series, even though I have basically no experience with the expansions. This game focuses on a few factions that are more distinct and fleshed out, like in other RTS games, but spices things up with the inclusion of mythical beasts and deities. For example, the Greek will have one of three primary gods (Zeus, Poseidon, and Hades) who will grant access to different unique units, techs, and bonuses. Over the course of the game, you'll pick new minor deities as you advance, meaning you can still reinvent yourself as needed for a good portion of the game. And the addition of mythical beasts and heroes creates a new layer of strategy in army building and tactics. Also, this game has the best campaign of the bunch.
Age of Empires 3 - Not the best game in the series, but it benefits from being the most modern. And despite often being overlooked, it has a few fun twists. The focus on the European colonization of America creates an unique dynamic where you can call for aid from your home country in exchange for activities like trading. You can recruit native tribes, which is both useful for your own growth and a new concern, since enemies can recruit them first. Unfortunately, the game's setting and style robs it of some of the epic atmophere in older games.
TLDR: AoE 2 and Mythology are great, AoE3 is good, the original is bleh.