Seating
 

I would like to book a cruise and be sure that at every dinner i will have a table for two in the main dining room. Is it possible? Wich cruise line?

Your best bet is to Ask your TA to request it which may not happen. If that happens, as soon as you board, go to maitre D & request same.
No one cuiseline is the best IMO on this.

Avoid NCL, which has no assigned seating. Otherwise, almost all the cruise lines have an assigned seating option. But now that many cruise lines are moving toward open seating, I'd heed Rhea58's advice. You might need to look a little closer at what is available at the time of booking. Just be sure to use a travel agent who will follow up to make sure you get what you want.

For example, I cruised on HAL in October and the reserved seating was not available when I booked in August on Expedia (I did my own booking online). Only half the dining room has assigned seatings; the other half is open. However, my longest wait time was 20 minutes on one evening for a table for two. Otherwise, I either never waited or waited for less than 5 minutes. So it just wasn't an issue. I've had similar experiences on NCL. It's just never been a problem to get a seat at dinner, so I've never been really annoyed at any kind of wait.

A couple of years ago, I took a cruise with my partner and parents on NCL. We ate at the specialty restaurants a couple of times; we had to wait on only one occasion to eat in the main dining room for about 15 minutes.

I usually cruise HAL and book early. I have always used cruise connections so my agent there books our preference of table for two late seating. We have had it every time. Late seating is easier to get what you want as main (first) seating is the most popular. Also they are able to arrange a table for two more easily than a large table so your chances are pretty good. Just be sure to request that as soon as you book.

Tipping extra to the maitre 'd on some cruise lines does wonders.
We now only cruise on ships with open dining and have never had a problem (or need to tip) getting a table for 2. We find that it is easiest if you sit before 7PM. After the first night or 2, we don't even have to ask, we are "automatically" taken to a table for 2.