Been working on my guild for some months now. Would probably have reached 25 by now, but Diablo caused "some" downtime :p

Anyways, I do my grinding on the first two bosses of normal Lost City (General Husam and the croc), which take a little less than three minutes total for me.
Normal 85 bosses give 31000 exp per char, so that's 2x5x31000=310000 per run.
To reach the daily cap, it takes 7807500/310000=25,5 runs, or about 75 minutes time (for me).

That's a bit over 100k/min. Comparing to 5v5 arena, unless the queue time + match takes less than 7 minutes total (last time I did 5v5 the queue alone was nearly 10 minutes), what I'm doing might actually be a bit faster - or at least more reliable. Losing just one match would cost you several minutes, so unless you're some PvP god who ALWAYS wins, I think there's no competition as to which is faster :p