It depends on what finish you are working on and what type of polisher you are using.
#2 (fine), #1(medium), and #4 (heavy?) are meant for rotary buffer's only, but some people have had success with these cleaner/polishes by DA polishers. These cleaners more or less concentrate on removing dead paint and oxidation over swirl removal (which these cleaners still remove swirls, just not as good as a polish meant for swirls, but a polish meant for swirls doesn't remove oxidation as much, very minimal difference in formulas between the two.) This is apart of the Mirror Glaze line.
The Meguiar's #9 SMR 2.0 is a very gentle swirl mark remover. Tt does contain fillers so you are removing/rounding out some swirls and filling the deeper ones.
DACP is a good all around cleaner and polish. It removes swirls and restores gloss. Its apart of their Body Shop Line.
Machine Glaze #3 was meant for rotary buffer application but some people prefer to use this over #7 Show car glaze because of easier removal.
Meguiar's #7 is a nice product if used correctly. Link below to directions on how to use it.
http://www.autopia.org/forums/showthread.php?s=&threadid=23740
Meguiar's #5 New Car glaze is not often discussed here. Mike Phillips can offer some insight?
While on the topic of glazes, Meguiar's #81 Hand polish is another glaze Meguiar's makes. Supposed to be designed for clear coat finishes (#7 is meant for older laquer/enamel finishes but can be used on clear coats too.)
Ok, now All that is thrown into the post, I recommend DACP and SMR as a good starting point.
If you are using the PC, the medium cut #1 is the harshest I would go in terms of liquid polishes/compounds.