6P is probably #1 with the G2 and E2e right behind. I think just about everyone on this forum who has more than 10 lights have atleast 2 of the above lights.
On this forum, the E2DL, E1B, A2 and L1 seem to get the most discussion. I agree, though, that the 6P is probably the most popular by total sales figures.
the most popular for me is definitely a C2 and/or a C3 with a P90 lamp assembly. They both give excellent light, the only drawback is, with the C2 you only have about half an hour of runtime with 2X R123s in it. That's why I like the C3 equally, although it's a bit longer: this one lasts nearly an hour with 2X 17500s in it...
Locutus (or should I say Captain Picard...), someone already started a virtually identical thread on this earlier this month, so the moderators will probably merge them or lock this one (or both).