I would say the biggest factor in whether or not you win a game is player experience. Experience with the game, and even experience with a specific list you are using, can make the difference beyond anything else.

That's not to say that if you just throw a bunch of random things together and play it all the time you are going to win. There's still strategy in list building, but it's not "go copy so-and-so's list", it's more about finding out what questions you want your lists to ask and answer (and there are many ways to ask or answer the same question), like "can my opponent handle heavy armor skew?" or "what do I have to beat high defense?" or "how do I deal with a debuff heavy faction?".

To give you an idea, this has all the tournament winners from pretty much everywhere. While there are some pretty popular models, you should note that it is not very common for winners to have the same lists, even if they used the same Warcaster.

Some casters are definitely more popular than others, but that doesn't necessarily mean they are more powerful. What it usually means, is that their power lies in something very simple and obvious, making them easier to learn. Some casters require a bit more finesse, and are less forgiving to new players, but in the hands of someone that knows what they are doing, they can still beat the simpler casters. Also, there are some casters that are not used simply because they've been out a long time, and people are tired of playing them. New models are always going to be popular in lists because they are new.

There's other things that can influence a game, like crap luck with your dice and fighting an opponent who is exactly not the list you were prepared to deal with, and sometimes those end up deciding the match, but mostly that's when the two players are reasonably close in experience.

Also, all of this only matters in higher point games, like 35-50pts (which is not what you should start with). Lower point games are much more about whether or not you happen to have something your opponent can't handle, or have a way to handle them.