I'm trying to keep myself from buying every expansion to every game I get (Carcassonne taught me not to do that). Is there one map that stands out as a winner? We'll typically have 4-5 people playing. Is the Robots expansion worth a purchase?
I'd tell you to pass on robots unless you really want to pad 1/2 more player than you have people for. It generally is somewhat slow to manage, doesn't make logical choices (causes it's just conditional cases), sometimes cheats, and is easily gamed. If you want anything out of PowerGrid that isn't maps, I highly recommend the new Powerplant deck, which feels more balanced and is more interesting than the base deck. I'll go through the map packs (each pack comes with 2 maps):
France/Italy - definitely the weakest of all - Italy's shape makes positioning interesting (and more scares starting resource count); France has Paris as "super city", which, again, leads to early positioning strategy gameplay.
Brazil/Spain + Portugal - An interesting variation on resource distribution - Brazil is heavy on Garbage (Biogas), and this skews the power plant distributions; Portugal is Nuclear free, so anyone starting in it can't bid on Nukes, whereas in Step 2, Nuke prices drop off dramatically for an interesting variation.
Benelux/Central Europe - A shorter variation of the main game - Benelux is a much smaller map, and results in a faster game; Central EU has 3 countries that are anti-nuke, so similar to the Spain+Portugal setup, with other oddities sprinkled in the map.
China/Korea - Definitely the biggest change out of all the maps - China is a structured growth, all plants are sorted in order, but there's always one less plant than # of players so it always creates a bidding war. Resources are plentiful until step 3, which forces people to plan correctly or be left without ways to power cities; Korea has a "2 market system", where players can only buy from one or the other - Coal is dirt cheap in the north, but they don't have nukes, which forces players to strategically think about where to buy from.
Russia/Japan - Somewhat odd map variation - Russia has a pretty weird modification for plant bidding (and slightly less plants to bid on); Japan's map is narrow, which has limited cities and prevents players from certain areas in different stages.
Quebec/Baden-Werttemberg - Most recent set of map, and limited to one print run - Quebec is heavy on "hydro" power, and has a very centralized map layout with the two main cities taking up 7 spots; Baden-Wuerttemberg is another small map, and the game turn order is reversed which forces people to strategize differently.
With our group, Korea has always been the standout winner, whereas the Brazil/Spain + Portugal, Russia/Japan pops up once in a while.