The question of who to trust with the captain's armband, and the siren prospect of double points, is a weekly torture for the FPL enthusiast, and one that tends to dominate the online forums in the last days before the gameweek deadline.
I actually dislike the 'double points for the Captain' rule - because it is just a further randomizing element in the game, which is something I believe we can do without.
There are a few important things to remember about selecting your Captain for the gameweek:
a) No-one KNOWS who's going to get the highest score in any given gameweek. So, it's fruitless to ask other people for suggestions. And the various online 'predictor' services that purport to tell you the 'expected points' for leading players are just guesswork; all the ones I've ever checked out have been laughably haphazard and inaccurate - far less reliable than even my own fallible intuitions.
And....
b) You're going to be 'WRONG' in your pick more often than you're going to be 'right' - you just have to live with that. If you have a decently strong team, there are almost always going to be at least 5 ot 6 players in any gameweek who should have a good chance of picking up high points for you. And it is impossible to know for sure which one of those will come up with the highest return. (Also, in many weeks, you'll be surprised by someone else in your squad, outside that core group of most promising picks, - perhaps even someone you left on the Bench! - coming up with a huge return, contrary to all reasonable expectations. Fate is cruel and mischievous, and loves to scoff at our puny efforts to prosper!)
And....
c) Even if you are fortunate enough to give that armband to the player who comes up with the highest score in your team for the gameweek, he's still quite unlikely to be the 'Player of the Week'. We so often see the 'Team of the Week' chock-full of randoms: in many, many gameweeks, a bunch of completely unexpected players - that almost no-one would own for FPL - produce higher scores than almost all of the more fancied players. So, that's a further cup of gall you have to sup from, even if you've done 'well' in your armband choice. (For the first half or so of last season, Mo Salah was producing high scores so frequently that he was returning on the captaincy nearly every other game on average - and was the highest scoring player overall in around two-thirds of those. But that is yet another measure of what an utterly freakish season he enjoyed last year; I don't think anyone else in FPL history has ever come close to that! And note, that run didn't quite last all season. And even during the super-hot spell, he was usually outscored by far more by the top player of the week when he had a poor return than he outscored the next best by in his good weeks; so, it was theoretically still possible - though very difficult [this really was the one-in-a-million instance where choosing the same captain every week for a long run of games might have been justified] - to get more captaincy points by consistently betting shrewdly against Salah than by always betting with him.)
Of course, the range of sensible choices for the captaincy each week is somewhat constrained. There are those few exceptional players like Haaland and Salah who produce very high scores more often than any other player - and, when they're playing well, 'blank' much less often than most others. And there are other outstanding players who may get close to those levels of return for shorter spells, when they hit really good form - and/or have particularly favourable-looking fixtures.
Form and fixtures, alas, are not entirely reliable guides to performance. And, as I just noted above, you almost always have at least 5 or 6 good players, who are hopefully in good form.... and facing fairly good fixtures.
But even your Haalands and your Salahs don't always produce a big haul against a 'soft' opponent. And 'form' can evaporate in the blink of an eye. There is really no way to tell who's going to do well or not so well in the coming round of matches. Even the Haalands and the Salahs 'blank' at least 1 game in 3, often more like 1 in 2; the majority of top players blank more like twice in every 3, or sometimes maybe even nearer 3 times in every 4 games.
So.... your captain pick will probably fail to return anything nearly half the time, even if you're doing really well in your selections. And he's only likely to be your best pick about once in every 4-6 games,.... and the top points-returner for the gameweek even more rarely than that.
These are the harsh realities you have to learn to accept. If you allow yourself to expect - or even hope for - a massive haul from your captain every week, you are going to be bitterly disappointed more often than not.
You should be grateful for any kind of return from your captain, even a modest 5 or 6 points (points earned from the gameweek action, before the doubling). If he scores 8-10 points, you should be very, very happy. And a high double-digit score is something that's only going too happen a handful of times a season at best - perhaps only once or twice, perhaps never.
Be realistic about this: selecting your Captain is a cruel lottery - and most of the time, you're going to wind up with a losing ticket!
[I followed this post up with a couple of afterthoughts on how to choose your Captain - here and here.]