Hmm. This is kind of a personal question. How much time do you have? Do you want to adopt someone's tried and tested build wholesale or are you more interested in the ins and outs of why a build works? Personally I fall squarely in the latter camp. Due to this being my first MMO and playing since 2015, it never occurred to me to do anything but learning the game organically myself.

Personally, then, I'd steer clear of complete pre-made builds, be they from Alcast or anyone else. Build videos make for easy content. It's much more difficult to lay out all the interconnections in buildcraft. It's also hard to be conscious of your own biases and playstyle preferences. As a content creator you can also take your own proficiency for granted and misjudge the need of a beginner, notably in terms of how much mag/stam sustain they need.

I find Xynode Gaming comes up first in Google / YouTube searches when I'm looking for dungeon mechanics, probably with good reason. So that's where I go for that.

Skinny Cheeks to learn about the endgame trial meta.

Malcolm for PvP magsorc advice specifically.

Hack the Minotaur, I think, does a fair bit of middle game solo builds, as do some other creators, such as Dottz, perhaps.

The UESP build editor is invaluable to create builds and review the resulting stats, without having to invest into or farm sets in game.

The Real Godzilla is starting to produce some general PvP educational videos.

I'll throw in some of my own advice. I solo PvP, solo PvE and, lately, normal / vet dungeons and normal trials.

Where to start? I think some classes, notably templar and nightblade, are very direct. Skills are situational and straightforward. Simpler than some others. Both magicka variants have skills that do damage and straightup heal you while doing that. No delays. Not stupid faffing about. Both have class executes and gap closers. I love those classes for that reason. They make you focus on the fight, rather than wrestling with your own class (mainly thinking of the magicka variants). Of the two, magplar is the easier in high-end content. I find it much harder to get good damage out of magblade. Puncturing Sweeps is unbeatable for ease of use, although I do not find that makes the class boring. Quite the contrary.

Magden is a class that is the polar opposite to magblade and magplar, in my opinion. Everything is about your rotation. You are quite safe, but you have no execute, no gap closer, and you basically do the same thing in every situation. It's a boring class for me. Mag DK is, perhaps, more interesting, but it's delayed heals generally do my head in. I don't like it. Magsorc is something I've played little of. I think it plays quite differently depending on your use of pets, or not.

If you do a lot of solo play or with random / uncoordinated groups, and you want to climb up the ladder in difficulty, you want self-healing while doing damage. Every class has that, but magplar stands out. Still, there are differences. All things being equal, I find magblade squishier than magplar, but a stamina Brawler build beats everything hands down. In general, shielding builds increase your tankiness while also doing good damage. Builds that produce shields while doing damage, not builds where you have to spend time casting shields. This can be achieved via armor sets, enchants and, perhaps most notably, the 2H Carve skill, preferably morphed to Brawler. A Brawler build with the Dragonstar Arena 2H weapon, Pale Order and good sustain is arguably the most OP solo build you can run in PvE. It has a a ceiling, because it's melee (inherently more danger, only offset by the shield), and because it's single-target damage is not great, if you use Brawler as the spammable. It is neither meta in trials nor for soloing near-impossible feats, such as vet Scalecaller. However it is one of the most comfortable ways to do, for example, vet Vateshran.

That said, the above analysis is rooted in my playstyle preferences. You can't get away from that. I'm primarily a melee player. I rarely play ranged. I find melee more exciting. I'm also prone to dodge rolling to avoid damage and reposition myself. I like high movement speed. This means I'm probably missing out on options that are equally good. For example a pet sorc is renowned as an easy solo build.

In terms of becoming somewhat competitive as a DD in content where that matters, let's say in trials or for burning the last boss in vet Dragonstar, I find magplar hard to beat. I find it the easiest to get good damage out of, while also being somewhat tanky, due to the self-healing from Sweeps. You ideally need somewhat exotic gear, though, notably the Kilt and Minor Slayer / Crit sets. With those, Sweeps alone does a ton of damage.