Oh ! I think I can help !
Here is the seating chart for Garnier next year :
and the prices that go with it
The rules of the house are : even-numbered seats are on the left half of the chart, uneven-numbered ones on the right side.
For the orchestra, balcony and amphitheatre : the lower the number, the closer to the stage you are. You don't have any indication of the row. (for example, Amphitheatre 17 is on the first row of the Amphitheatre)
For the boxes (loges), you have first the floor, then the box number, then the seat (ie 3emes loges de côté 16 1)
Again, even-numbered boxes on the left side, uneven ones on the right side, starting from the stage.
Inside each box, seats are numbered : first row 1-2, second row 3-4, third row 5-6. Seats 1, 3 and 5 are always those closer to the center of the house. This is not important for the first row, but it does make a difference for the third row as boxes are separated with a wall, especially if you are in a side box.
I obviously haven't tried every seat, and some people may have other preferences, but here's what I think :
Categorie 1 : in red on the chart (plus Optima, but this is not used as of yet for ballets) : it's where you have the most seats, and the best ones, if you are ready to pay the price. As said, the orchestra (stalls) lacks a bit of rake, and the seat are aligned so if you happen to be seating behind a tall and bald man who likes to seat very straight (like for example my father), it can be unpleasant. The balcony seats do not have this problem. The first row seats in the boxes (loges) are excellent, if further away, and you are a bit high in the 3d loges (this year those seats were in second categorie).
Seats to avoid : second row in the boxes. You pay the same price as those in front of you, but are seated behind them.
Categorie 2 : in darker blue. It includes foldaway seats in the orchestra and a lot of second-row seats in the boxes, which I try to avoid as a rule.
I also avoid the baignoires, whatever the price.
Categorie 3 : in yellow. Again, avoid seats 3-4 in boxes. Be also careful of the boxes in center (above number 25-26) : you'll be at the back, with 6 people before you.
First rows of the side boxes are alright.
Categorie 4 : in light blue. Most of them are in the amphitheatre, which is, as said, very uncomfortable : narrow folding seats, without a back, no room for your legs, but at the same time seats are high so your feet may not fully touch the floor. And it can be very hot. Best seats in the categorie are 3d and 4th loges on the side (the higher the number the better)
Categorie 5 : in purple. If you don't mind standing, those are the best seats for the price : third row in 1st, 2d and 3d loges, first row in 4th loges. Avoid second row of 4th loges, you'll have to stand, but without support.
Categorie 6 : in green. Not sold on the Internet. Restricted view everywhere. A few seats are good, but good luck getting them. Do not buy "stalles" or 5th loges for ballet.
Boxes to avoid : anything not first row in boxes 23 and 24 : they have a column that restricts the view a lot.
Do not buy tickets from resellers in front of Garnier : black market is illegal. It is legal to buy and sell tickets as long as they're sold at the indicated price or cheaper. The Opera does not take tickets back, but has a secure website where you can sell (for free) or buy (with a commission) tickets here
The website is awful, especially compared to the new ROH one, and you cannot choose your exact seat, merely the section and categorie.
Be also aware that not all seat are sold on the Internet : it is always useful to call if you don't find anything to your satisfaction on the website. A lot of times seats are also made available later, so it is always worth checking the website regularly. Of course, the closer you get to performance day, most seats will only be in categorie 1.