If you have an affinity towards racing games (yes I'm aware this game's racing mechanics aren't exactly well-developed), you can get an easy 20 tokens in roughly 2 minutes.
If you have a MechroMancer with Close Enough 5/5 & The Bee (or a crapton of Anarchy Stacks), you can get 25+ tokens in 1~2 minutes (depending on your luck with spawn patterns of the SuperBadass's you need to kill).
The arena fight is easy enough, but I don't feel it time-efficient for gather tokens as fast as possible.
red_ghost from Monster Hunter / Phantasy Star Universe
Bar room ball. Tested and it's faster. The quickest I was able to get 20 from the race was 80 seconds or so. 1:45 to finish the race and I had something like 20 seconds left over. Bar room brawl on the other hand, you get 2:30 (I think) and the fastest I was able to do that was in 45 seconds. The trick is to stay in two areas. The Badasses only appear to spawn upstairs and out of the top right door on the bottom level (according to your map) so all I do is stay close to that staircase and travel up and down between floors. Never have I failed to kill them all this way and usually it's in under a minute.
Intersting note, if you throw teslas in these areas the arcs go thru walls and the floor of the second story, so they'll clear the whole area. Me, I do the race w/afterburner relic, 20 every 3 minutes, and it's free with no rats.
fill a game w people who have a few tokens, take turns hitting the slots @the bar, made 500 in like 30 minutes last night on 2 toons while my friend had 2 of his in game also, cakewalk, just watch those mirvs.