I'll add
www.boardgamearena.com since it does have Talon, Space Empires 4X, and Unconditional Surrender on it. The recent additions of SE4X and Talon have me hopeful more things will end up on BGA. And it's generally a very solid interface with a lot of games to choose from.
As Brant mentioned, Rally The Troops is another good site to check out. I've only poked around on it a little bit, but it seems really solid.
I've used both Tabletop Simulator and Vassal. I cut my teeth during covid using Vassal, so it's my preferred way of playing. I'm sure if I had done so with TTS that would be my preference. Regardless, both offer a wide variety of games that you can play. TTS is a bit more of a physics engine, whereas Vassal is a bit more of just an interface for playing games. My experience with both has been it depends on the module. With enough know how, whoever created the module can do different amounts of scripting. I believe TTS is more powerful there.
I play a lot of Combat Commander via Vassal and it's module is pretty good (I say that as the person who has kind of taken to maintaining the module to whatever degree I can--if I knew how to program I would work on adding more and better features to it, alas that's beyond my abilities). By far the most impressive Vassal module I've used is the one for Pax Pamir 2nd. I only played a few things via TTS. The scripting there was pretty impressive for War Room.
For both TTS and Vassal, though, in my experience it falls on the players to make sure rules are being followed. BGA and RTT have rule enforcement built into them.
I know Vassal has the ability to save and email games, if PBEM is your preferred format. I think TTS can do that but I'm not 100% sure.
Finally, depending on the game(s) you're interested in, there are some with dedicated communities. I play a lot of Combat Commander because I'm part of the ladder tournament group. I know of other groups for Command and Colors, Last Hundred Yards, Fighting Formations, and Wing Leader. I'm sure there are more.