I've been experimenting with this for years now, and with Julie's help as co-taster, I have arrived at a few ideas. We slowly added and left out many of the spices to see what is the best for our taste. We don't use sweetener, like honey or sugar. Indians have their tea (every type) very sweet - as a culture they haven't yet moved away from sweetness in their foods.
Cardamom is the main ingredient - you can leave all the others out, but not this one. I prefer ordinary Cardamom, but Indians always prefer Black Cardamom - apparently it has more aroma, but is bitter so you need sweetener if you use it. You can use a lot of Cardamom compared to the other spices, so the idea is to reduce it to its necessary level as it is not a cheap spice - Indian shops often have it much cheaper.
Allspice: I finally decided this is a good addition. Just a small amount - I use only three seeds.
Cloves: I have experimented with more and less and none. It adds bite, so I finally decided for an unsweetened version to only use about two cloves.
Black Pepper: actually I experimented with all kinds of pepper, but in the end decided black was best. I would not leave it out as I like it's different bite. I use three or four.
Anise and fennel - much the same flavour and I haven't used either - not sure I want to but many recipes use it.
Coriander: due to its common inclusion in recipes we tried it, but finally decided against it.
Bay leaf: again in recipes but I don't see the point - I may try it next time and let you know what we think.
Liquorice root: I like this as it adds a malted flavour. I only use a small pinch.
Cinnamon: We went from two sticks to none and finally settled on a half stick.
Cassia bark: We used it till it ran out and after that didn't notice much difference, so we leave it out now. If you have it, just a little.
Nutmeg: If you can be bothered it is always a pleasant addition, but we can't be bothered. I also wonder if you do use it, perhaps it may be better to add after the heating process as it seems to quickly lose a lot of its character when cooked.
Mace: To be honest I don't know what this flavour is. I must get some and experiment.
Vanilla: Many recipes include and I think it would be good even though I don't use it, but the best inclusion is a full vanilla stick if you can afford it.
Ginger: We always use fresh grated ginger, and it makes a difference when you add it - earlier to the heating, the less bite, but we prefer it that way now. I use a small handful of grated ginger.
I use only full seeds, then grind them all together in a spice grinder - the finer you grind the less you should heat. I prefer a course grind myself.
I put in pot and add boiling water, then bring to simmer for 5 min, pour into tea pot with three tea spoons of tea for a two or three large mug quantity of Chai. I have tried numerous variations from boiling for 10 mins to simmer for 5, and have finally preferred the quicker process, although I don't strain into tea pot - it all goes in.
After 3-5 mins in the tea pot, stir, then leave for a minute to let the stuff fall from the spout opening, then pour with generous quantity of milk in the cup. Indians actually brew the milk with the spices and tea, but I have never perfected that method to my taste. If you normally use sugar in your tea then add the same amount - we never use sugar in tea, as it kills the flavour of the tea, as does milk, but milk with strong flavoured tea is very nice.
Oh, and one more tip - double the people is NOT double the spice! Just a smidge extra, or better, no extra as most new-comers to home-brewed masala chai are not used to its flavour so best start very mild. Commercial masala chai is quite sweet and insipid. I haven't found a commercial blend yet that I like after brewing my own, but I admit, it does seem to be an acquired taste.