Without a resonator at the moment, but I keep a flat-top set up for lap-style. I really like open G (DGDGBD), but generally increase the string guages and tune it down to open F (CFCFAC) or open E (BEBEG#B). The lower tunings sound a little "darker" to my ears. The cool things about this tuning are the groups of strings (2,3,4, and 1,5,6) which are still tuned at standard tuning type intervals, so a complete rethink isn't necessary to get started.

I also like D'Addario FlatTop strings for slide because they are slightly flatwound and the slide moves cleaner for me.

Btw, I use the same tuning(s) for regular (non-lap/slide) playing too because, for fingerpicking, I can get timbres and harmonies which wouldn't occur to me in standard tuning.

Peace, Mooh.