Essential Guitar Technique
I am a big fan of the “max grip” series of picks by Dunlop.
As they are made from nylon, they give a great tone, and the extra grip helps to stop them slipping in my fingers.
In the past, when I used smoother tortex picks, I would be happily strumming away and then, the next thing I knew, the pick would be facing another direction!
In terms of pick thickness, feel free to go as thin as 0.4mm, although this can be far too thin for some (they are good for beginners or those who struggle to strum though).
Usually, for acoustic strummers, picks from about 0.5-0.8mm work well. My standard choice is the Dunlop Max-grip 0.73
You can go thicker than that but if you do, you may find strumming tougher. Thicker picks are good if you play a lot of single-note faster runs, which usually suits electric playing.
I actually use the Dunlop Jazz III for electric playing which is 1.38mm thick (don’t let the name put you off though, I use it for rock).
There are other brands of course too, so feel free to experiment. Anyway, those are my thoughts on picks and some of my suggestions.