by lithy
One note, On the Brink is still somehow between printings and aftermarket prices have pushed over $100. It is supposedly 'At the printer' according to Z-Man's website but that has also been the status for at least 6 months, maybe longer.The easiest way to describe On the Brink is 'Pandemic but moreso'. It will be very familiar, doesn't change the game as much as add to it. More roles, more events, a couple new modes.
On the Brink is definitely worth getting for the $30-35 retail price. Not so much for $100, and since In the Lab technically requires On the Brink (although you can work around it) and it changes the base game quite a bit, that one is out. State of Emergency is probably the weakest of the three expansions, so that leaves me with a different suggestion.
If you just want 'Pandemic but moreso', I would suggest either Pandemic Legacy: Season 1 or Pandemic: Iberia. Both are fantastic and both are standalone games, Legacy is obviously a one time playthrough, you will play 12-24 games (more likely 15-18). Iberia come with two variant modes right out of the box as well for some extra variety.
Games like Legacy: Season 2 and Rising Tide would be if you are looking for something a little less familiar as Pandemic.