To quickly gain RP(requisition points) and also advance your team well. i suggest farming Act 1 Mission 2 named "Infection".

You will not earn RP as fast as other missions can net you, nor will you gain huge chunks of XP by sitting and clicking for hours in 1 mission. (but this is good as you get lots of RP and packs whilst also leveling up your lowtier marines)

once your marines are in position nothing can go wrong.*
you will net 3k xp on normal and about 4k xp on veteran, per marine. thats 12k on normal and 20k on veteran nicely divided among the team. (i have yet to find a mission that nets more and spreads it out so nicely)
by speeding up gameplay you can finish the mission in 6.5 minutes.

drops are semi useful:
impedance field lvl3 drops a lot
darkangel plasma gun
mastercrafted plasma gun
mastercrafted plasma pistol
plasma pistol
and on veteran also urus word bolt pistol. (frequently drops, and its very good if your assaults have high accuracy, not that they should...)

not a level for farming items but some are nice and selling is always the same
it will let you quickly rank up low level marines whilst buying packs more efficiently than other farming guides.(sort of 2 birds with 1 stone here)

setup like this in second turn
you only need 2 good tactical marines to hold 2 points and can use the other slots to gain xp for new marines you wish to level up.
you should use a mediocre marine with good range in the position second from left to cover center.
your close range guy you want to level in the middle and your longrange guy on the far right covered by a tactical.
a strong tactical should hold the leftmost position.
overwatch the square below your marine, no need for diagrams where to aim, just aim directly down.
as you can see a bad run with 1 marine doing more work still nets a average of 4k on veteran.

You may disagree but to me this is the fastest way to farm between missions and level up new marines as i go through the campaign.

*the author of this guide is not liable for anything should anything go wrong. following this guide automatically assumes you understand this.