Whatever works for you and depending on how much information you want. GTs tend to be more general - an overview of your life over a period of about 6 months to a year - although you can specify something different and answer specific questions.
I like 5 card readings, especially for most quick questions, yes/no questions and finding lost objects. I read them in a line, starting with the first card as subject, rather than radiating out from the center - but whatever works for you.
The Square of Nine is great if I want to explore a specific issue in a little more depth.
Personally, I perceive all of the above as segments of the Grand Tableau - I see it as cutting into a cross-section so that I can focus more on a limited range of cards.
This is pretty much how I see it. It really depends on the purpose of the reading.
I like the 5-card line for simple questions (and have been experimenting with Caitlin Matthews' playing-card "suits and pips" material to augment the "yes/no" angle). I'm torn between the "all-in-a-row" approach to reading them and treating the center card as the focus. The former lets you bring a bit more of the "distance" technique to bear, while the latter seems most appropriate when you pre-select a topic card, find it in the deck, place it in the center of the spread and pull the cards on either side for the "before-and-after" sequence. Since I tend not to pre-select a topic card and just let the cards do the talking, I'm leaning toward using the randomly-generated first card as the subject or focus, with the rest as modifiers. I've found 3-card lines to be too truncated, and lines longer than 5 to be creeping into "nine card square" territory (but without the combinational flexibility).
The GT is the most inspiring, informative and fun to explore, but it is also the least used with my regular sitters because of the lengthy span of its outlook. I also recognize that it is the backbone of the entire system, as Mary notes.
That leaves the 3X3 square among the more usual approaches, and I favor it for its versatility and overall utility. It gives me all the information I need for single-pointed answers, and for short to medium-range projections. Some readers use the center card as the subject, other use the first, or top-left, card (again bringing the "distance" technique more fully into play). Here I go with the first option, but I'm open to the second one too.