Wouldn't any synth be a good synth for an intro
You can use a plucky super saw lead and slowly remove the high pass -> lead + build up. Sample something -> good intro could be used for the break in a drop.
For strings dsk overture is free and pretty good.
Could just use your beat or a version of it.
I like harmor a lot and use that quite a bit.
Sitrus makes some good pads. Fuck around with that shit.
Reverb on anything = intro pretty much.
Really who needs intros. Drop us in medius res like a true pro. stop pussy footing around