Curious what people recommend for picks, and how much of a difference it makes? I think I've mostly been using medium gauge, but these were all free picks and I find at times they like "flap" against the strings if I strum quickly making a lot of "ticking" sounds that aren't so great.

Figure on buying some decent picks since I've been playing for a while and they aren't that pricey, but can't remember what the difference is between light medium and hard, nor do I know of any specific brand/style that might be good. So any advice/info appreciated thanks!
i like ernie ball heavies. they're made of nylon and a stiffer than a medium but not super thick either.
Jim Dunlop Jazz III

Jim Dunlop Stubbies

No way they'll 'fap' on your strings.
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Under 1mm is generally for strumming chords. 1 - 2mm is for picking, as they're sturdier for that; and 2 - 3mm is for a warmer jazz tone. You can get metal picks for a really hard, bright sound.

They're pretty much all just lumps of plastic, but some have ribbed or matted sides for increased grip. Dunlop Jazz III's are revered by shredders.
You pretty much have to try them out on your own and see what feels most comfortable for you. The rules of thumb are a decent starting point, but depending on how you hold your pick and what you play, it can be very different.