Tower health: the regen is good early on in the game so that your front towers don't get whittled down; you'll have a tough time holding the flag in your lane if there's no tower nearby for cover and teleports. You may wish to get level 2 if the other team holds neutral portals, since those put additional strain on towers (regarding neutral portals, let the enemy have them early game so that you can get extra experience when they suicide on your towers).
Tower damage: I haven't tested the "splash damage" but this doesn't seem worth it.
Reinforcement health and armor: not worthwhile until you get priests (and if it wasn't obvious, it's mostly for the purchasable reinforcements, not the wimpy regular guys). Less worthwhile if the other team has good aoe. I don't like to go past level 2 because you can get a pretty useful item instead.
Reinforcement damage: usually more useful than reinforcement health, as long as you kill the enemy reinforcements so that your own spend their time attacking DGs and buildings.
New Reinforcements: priests are very useful, but I think it gives the other team more exp, so you don't want to rush them. Wait until you've spent like 3k on items, unless the other team gets priests before you were going to, then match them, since priests do push lanes pretty well. Angels are expensive and fly around too randomly for them to be worthwhile. Don't get them until you're ready for catapults. Catapults aren't very good unless you're still getting to enemy towers; if you're pushed back too far, spend your money elsewhere until you're at enemy structures. Giants are awesome, and where reinforcement health really kicks in; get them asap. Unfortunately, if the other team has Giants more than a few waves before you, you're most likely going to lose.
Mint: Oh yeah!. You can get the first 2 levels, but the 3rd I'm sure usually isn't worth it because games usually don't last long enough. If you're getting these, you want them early so that you end up making more money off them.
DG exp: I don't think this is worth it unless you've got extra gold to spend. With better items or other stuff, you'll be staying in lanes longer (run back for health/mana less) and getting more exp out of it anyways.
DG death penalty: I THINK (not positive) that I noticed recently that you don't get gold while dead. If that's true, this MIGHT be worth it because besides getting back in the action slightly faster, your team would be losing out on less gold (especially with mint upgrades) when they die, which would add up.
Building Upgrades: Trebuchets I don't feel are worth it unless you've got fortresses within range of enemy structures, because they can take them down at a decent rate. I don't know the exact mechanics of the Finger of God, but it seems too random to be worthwhile.
Hope I'm not forgetting something.
So pretty much my general recommended order would be:
tower health 1, mint, priests, reinforcement damage 1, reinforcement health 1, reinforcement damage 2, reinforcement health 2, mint 2, angels, catapults, giants
If your reinforcements are getting owned by DGs, get mint 2 earlier and reinforcement health and damage later. Throw items in there based on what you really need for your build, and when your warscore isn't high enough for the next upgrade. More money (from kills) should be translated into better items or consumables, not sooner upgrades, with the exception of mint.
Edit: I've found myself recently just rushing mint (currency) upgrades. Don't even worry about priests unless you're getting harassed a lot.
