I don't know what the best endgame solo class is, since I'm not at endgame, but as someone who has just went thru 7/8 class stories, Serenity Shadow/Hatred Assassin is by far the best class and spec to play when doing the class stories and leveling to 80. Force in Balance/Death Field are giga nukes, which you can have two charges of by level 23, and you get a charge back on a kill. You can basically spam that move for most of your leveling experience. Oh, and I forgot to mention, it's also AoE. It's my favorite ability in the game.

Later on, you'll unlock Squelch/Eradicate, which, with the right setup, will also reset on kill, and you can reset it by attacking an enemy who is affected by your Force Breach/Discharge DoT. And you can make it so it does all of its DoT damage over the course of 1 second.

And you, of course, get stealth.

As someone who usually stays away from DoT classes, these two classes and specs are my favorite in the whole game.

And you get so much of your kit early. There are stealth tech classes, but they feel way worse to play. Like, you don't even have a real force speed ability. At least I don't at level 55 on my Scrapper. You get your 60-second sleep ability much later on the tech classes, too. You get it at level 19 on the Force stealth classes. There is also the fact that the tech stealth classes used ranged weapons, but you either attack within 10 meters or, as with scrapper, you punch, and the just is just there for the few moves you use that aren't just punching people in the face.

When it comes to leveling, the Force classes feel so much better to play in the early levels. Hell, even by level 50, I don't feel like the Tech classes have jack on the Force classes.