As much as I love my smaller tanks, my favourites are my 5ft ones.
The water quality is a lot easier to maintain and you have a much wider range of fish to choose from.
I can't place a vote because it depends what I have in mind next. If I am going to breed small livebearers, a 10 gallon is a good size but if I want large cichlids or a big display tank I like my 120 gallon at 6 feet long. I have lots of different sized tanks for different purposes. Each one was purchased with a particular goal in mind and other sizes would just not be right for those purposes.