I only have OTB so I can only comment about that but you really should get it.
1) You get more roles. This lets you have up to 5 players and can let you have more variation in how you work to treat the diseases and find cures. 2) You get a 7th Epidemic. Just in case having an Epidemic turn up every 4-5 cards isn't enough 3) An extra disease (purple) with only 12 cubes available instead of 24. You must keep an eye on this little mongrel! 4) New Events. These are shuffled with the original Events and added, 2 per player, to the Player deck. You don't know which Events are available. No more relying on One Quiet Night to help you out! 5) Virulent Strain Challenge. Haven't played this one yet but it makes one disease tougher. 6) A bioterrorist. Haven't played this either, as most of my games are 2 player. This allows 1 player to make the other's game harder by deliberately spreading a 5th disease. Gives the game an adversarial challenge. 7) Petri dishes. Just cool and gives excellent storage for the disease cubes. My daughter was impressed with them and the labels (she's studying medical science at Uni and microbiology in particular.)
While I love In The Lab the most, you really should get them in order of release. On The Brink adds so much to the base game and the other expansions build off of it. In The Lab has the lab challenge which is probably my favorite, and most thematic way to play. State Of Emergency is good, and the new roles are nice, but it doesn't have the punch of the other two.
Personally, I have all of them and love them all (excepting the Bioterrorist and Team games), but you are best served getting OTB, then ITL, and optionally SOE.