>>534926297
>>534926892
Actually as I was leveling, out of boredom chose to determine the best/quickest way with some math n stuff. Obviously it's going to be co-op vs AI, wins get you more xp and that's the one way to guarantee it. Doesn't matter which mode, all give the same xp so you hit up intro bots (there's usually a MF bot mid and sometimes she ends the game early herself based on the enemy bots assigned), but overall, wins give a little less the longer the game, but its a negligible decrease:

12:41 win - 127 xp (0.166 xp per sec)
24:27 win - 237 xp (0.161 xp per sec)
30:07 win - 288 xp (0.159 xp per sec)

A lot of guides suggest to get wins ASAP, and while true, it also means you're going to go through the queue/locking in/loading in twice as many times, so to compare 1 long game vs 2 quick ones of the same length (remember we ignored the first 3 min queue/lockin as well):

13:00 min game (0.166) = 129xp
3 min break = 0xp - note, 3 min is pretty quick, I had much longer intermissions
13:00 min game (0.166) = 129xp [[258 xp total]]

29:00 min game (0.159) = 276xp

Also, day 3 you receive a 3day xpboost reward, if you go full retard hard farm you could get close to 30. Each level requires more xp, but if you want to get it done for free, you'll grind. Personally with this in mind, I prefer to extend games until that 30 min mark where sudden death activates, it requires less attention later on compared to early levels, and jungle is the best spot to go through this process, you just attack a camp and tab out if playing the right jungler.

TL;DR - co-op vs ai intro bots, jungle most braindead, the longer the games the better as less effort needed