this is a topic that has been discussed on BGG before but not on this blog, so i figured we should get the ball rolling! as with my other posts i will focus on USA/Germany and other maps with a similar resource structure.
so for those who may not remember, the possible starting plants are 03/04/05/06/07 in 2-player. 08 is added in 3P, 09 in 4P, and 10 in 5P. 6P is a bit less simple - usually you'll see the 13 last, but the 11 or 12 can replace it if it gets drawn that early. here's my attempt to break down the options in each size of game on "standard" maps:
03: 2 Oil -> 1
04: 2 Coal -> 1
05: 2 Hybrid -> 1
06: 1 Garbage -> 1
07: 3 Oil -> 2
08: 3 Coal -> 2
09: 1 Oil -> 1
10: 2 Coal -> 2
11: 1 Nuke -> 2
12: 2 Hybrid -> 2
13: nothing -> 1
2-player: real cut-and-dried, yet perhaps the toughest one to explain: bid the 04 (2C->1) up to $8-9, loser grabs the 03 (2O->1). 05 (2H->1) is likely a couple bucks cheaper to run than 03 for the first 2 turns, but building first is always worth more than a couple bucks. 06 (1G->1) is almost always the worst starting plant since trash is so expensive and turn order is mediocre. i have seen arguments for 07 (3O->2) over 03 but do not agree with them. 03 costs a total of $9 to make $22 on the first turn, and most likely $6 to make $22 on the second turn - total profit would be $29. 07 costs $16 to make $33 on the first turn, and almost certainly $10 to make $33 on turn 2 - that's a profit of $40, which sounds like a big advantage over the 03.
however, i think what you lose out on with the 07 is easily worth more than $11. you go second for first build and will be guaranteed the worse turn order for turn 2 since you must build 2 cities with this plant. unlike the 03, the 07 uses more oil than is restocked per turn on 2P. finally, the 07 has awful synergy with the available plants on turn 2; 03 and 06 are obviously horrible in this spot, but the big problem is that the 08 (3C->2) hurts you more than it helps. since you cannot build & power 4 cities this early with such inefficient plants, one of those plants would be completely wasted on this turn, allowing the opponent to take the 05 and make as much money as you while keeping their turn-order edge. the 07 forces you to take the 05 or pass on a plant altogether on turn 2, either of which gives the opponent their much-needed 08 at list price. 07 + 05 and 04 + 08 will both power 3 cities on turn 2, but the 04 + 08 player will have cheaper cities and cheaper resources. the 03's main advantage here is that it allows you to make full use of the 08, so you can get it yourself or drive up the price for your opponent.
in short: 03 guarantees the best real estate and gives you a full set of options both for the initial build (claim more territory or play for turn order) and for turn 2 (take the capacity lead or keep turn order). the 07 leaves you with no options and guarantees you will be in worse shape than your opponent for the near future - i'd say that's worse than losing out on $11...
whew! don't worry, the others won't be nearly as long!
3-player: 08 is clearly the strongest here since it's guaranteed to be sold at list price. build order is much less important than in 4P and larger games, so long-term moneymaking and options rule the day here; that said, 05's advantages over 03 are so marginal that 03 is still the best choice for the 2nd buyer. 08 > 04 > 03, plain and simple.
4-player: even though it now becomes more important to carve out prime real estate early, i think the 08 is still the best overall starter on the majority of map configs. you make more money on turn 1 than everyone else and can afford to pass on a plant for turn 2, giving yourself good turn order and a lot of money once the good plants start showing up. however, there are some map configs where being 4th to build is a major handicap; here i would pay more than $10 for the 04, more than $6 for the 03, and even a little above list price for the 05 if necessary. speaking of the 05, guaranteeing 3rd in turn order is what makes that the fourth starter worth buying along with 03/04/08. 07 may seem to have an argument somewhere here since coal's price is driven up more here than in smaller games...but it still loses value since using oil doesn't attack as many other players and the lower restock rate is a problem. the 09 also becomes available for the last player here, but with bad turn order and low capacity it represents the worst of both worlds.
the starting plants in 4P should always be 03/04/05/08, but their relative value can change quite a bit depending on map configuration. 08 should be bid up heavily on more "balanced" maps like Germany, and still deserves some competition on most other map configs. the value of the others depends on how many cheap regions are available in the given map.
5-player: usually, you really do not want to be the last player without a plant in 5P. sure, you'll get that sexy-looking plant 10 (2C->2), but building 5th will almost always be much worse than even 4th since having 5 regions tends to bring more expensive ones into play and 5 players makes things more congested. the 10 will also have its cost driven way up by all the other worthwhile coal plants in play. thus, you will need to fight harder for the 4 plants mentioned in the previous section. i could actually see an argument for taking the 07 over the 10 last, but i don't have a ton of 5-6P experience and need to do some more work on this...
6-player: pretty similar to the above. get one of the big 4 plants unless it will ruin your building. If the 5th buyer takes the 07 then the 10 becomes the clear winner for the 6th player. if the 10 is gone then the last buyer faces an interesting decision between the 07 and the 13 (_->1); i favor the 13 since you can just build 1 city and carve out good turn order for the near future (remember that building 2 cities at the start is usually best). if the 13 gets bumped off the market by the 11 (1N->2) or 12 (2H->2) the 07 becomes a no-brainer.
i have also heard an interesting idea of buying the 06 then not building on turn 1, but i wouldn't recommend it unless you just want to be wacky for a game!
that about concludes this mutha! hope y'all appreciate this - let's see some discussion!
EDIT: adding in descriptions of each plant.