IMO in one of the center setor grandstands, the best you can afford (just not the setor 9 tourist section). Maybe 7,8,10.
Note: The musicians parade and then tuck into a niche next to setor 11 for the remainder of their school's parade, which can be a bit loud for some.
From the grandstand, you get an overall view of the colors and costumes, and are at the level of many details on the floats. The last sections (highest numbers) often mean the schools are in a rush to get out of the stadium before their time is up. The beginning sectors (lower numbers are given to locals (the very first ones) and don't have a view in both directions of the parade pageantry. If you are alone or have doubts about keeping your seats in a large "first-come" set of concrete steps, bring some sort of portable foam stadium seating. However, most spectators will be on their feet during the parade hour.
I highly recommend doing as much research on the schools (history of each and their standings in this and past competitions), themes and songs before attending, so what you are watching is more intelligible.