Mmm, so, looking at the options there's a couple of general things we have going on. Firstly, Trixie is a Pikachu, which means that being fast and zappy is the name of the game. We don't want to get hit, and by and large we want to be zapping things from range. The second point, however, is that from Trixie's perspective the conflict is man vs pokemon rather than pokemon vs pokemon, and some of the options really tie into that more.
My least favored option is probably Electroball. It's a harder hitting attack, which is always good, but it's a bit whatever too and isn't really expanding our capabilities or options a lot. Thunder Wave is probably also lower on the list here - AoE paralysis is good to have, but we do have nuzzle already and it doesn't feel, idk, as tricky? Probably something we want to pick up eventually though, especially since it also initially tied into better charge.
Nasty Plot feels like it really works much better the more we've levelled up our other capabilities - getting Nuzzle II and Quick Attack II and Thundershock II etc. really open up its capabilities much more.
Thief ties into the man vs. pokemon conflict more - it's an anti-human technique more than anything. Could emphasise that aspect of her personality more.
Double team is sick and definitely something we want eventually as a Pikachu imo, but might be better to ensure our Quick Attack is up to snuff first? dunno. It's probably a stronger combat option?
So that leaves us with Nuzzle, Thundershock, and Quick Attack as I think my highest ranked options rn. All offer some nice capability enhancements. Thundershock is probably the biggest combat enhancement, leaning into the importance of good ranged attack for Pikachu, while Nuzzle perhaps leans more into doing stuff with human things? Quick Attack II is just cool as heck, and mobility is always going to be important for us. Making sure we can't be easily cornered and can get into places we shouldn't is always good. Lots of synergy with everything else, such as the close range shenanigans of Nuzzle and Thief, the evasive trickery of Double Team, and general kiting for ranged combat.
I guess one thing to think about is what our next move could be too...
Going something like Thundershock + Double Team would be a strong trickery-styled combat build. Something like Nuzzle II + Thief otoh would emphasise man vs. pokemon and interactions there. Different builds have different implications.