depends on how fast you can kill enemies/bosses and if you're playing solo or multi
ttf for multiplayer is prolly fastest, esp if you do the full run at a decent speed
dragon and de rol is pretty efficient for leveling, mines is only worth doing if you can do it fast or know you can do full run
for solo max attack 4 in episode 1 and 4 are also very fast, if you can kill enemies quickly
for ep1 ma4 b or c are good, usually just end before mines or full run (ma4c ends with falz)
ep4 ma4 b is good if you can full clear crater, but there's a shitload of zu
ep4ma4c is better if doing short runs or full run, just end 2 waves after the first warp
for norm-vhard dim/shadow/dark/hell handgun/raygun are great for leveling, especially if you still dont have a good mag yet
mop up 3 is prolly the fastest quest in game for exp, but you need to be able to kill mobs pretty quick and not get hit/die instantly
since it's a mob grinder you have to tag all the enemies to get the exp, usually quest reserved for human/newman or forces that can use techs to tag/kill
it's ok on cast if you have good equips already
http://www.pioneer2.net/community/i...-question-quick-answer.306/page-36#post-11545
for future questions, people usually respond pretty quick