Unfortunately, there has never been one universally popular set of 'big battle' rules for Napoleonics in the same way that WRG was 'the' set of ancients rules for many years - which makes it difficult to recommend a set for 'pick up' games: just use whatever everyone else is using locally!
So many Napoleonic games seem to take forever to play, never reach a conclusion and give you a headache with far too many tables and complex mechanisms.
The best I've found would, again, be Shako, or Age of Eagles (based on Fire & Fury) both of which have simple but subtle mechanisms that allow you to play either divisional or corps level games within a reasonable period.