These days I would say the Godox V860II Li-ion... but it's a bit of a beast (big/heavy, similar to SB910). For smaller and lighter probably the SB700 although it's a bit less capable and more expensive. Even cheaper than the SB700 would be a used SB600 or SB800, but the user interface is clunkier.
I do think everyone should have a speedlight, but if the goal is to get into more advanced lighting it might not be the right choice.