I play support, with a little bit of jungle sometimes. I play these roles in part, because I can't farm. If I wanted to learn how to do that, which role would be best? And are there any particular champs that have an easy time farming?
I think its best to learn with Champs that do the most damage: AD Carry and Midlane. As AD Carrys find one who builds Runaan‘s Hurricane as second items. Easy Waveclear. In the Midlane Artillery Mages have the best waveclear and some burst mages like: Xerath, Lux, Orianna, Ahri, Diana and Syndra. But honestly just go into practice tool with no enemies and just last-hit minions to learn it. Thats probably the easiest way to learn it. Then you can farm well on all champs :)
are pretty good for practicing last hit be it their passives or abilities
ADC is the easiest to learn how to farm, where as top lane is basically the advanced farming mechanics; when to trade, wave management etc.
Midlane farming varies heavily, and is basically "can i burst my entire wave? Can my opponent burst theirs?"
Doesn't depend on the role, depends on a champion you play and how much damage they deal to minions(special damage multiplicstors or reduced damage). 
farms better while 
can't do crap against minions early on.
It also deoends how glld champoons fsrm in all stages of the game as well as how good they are at sieging super minions(again, Kai'Sa wacks suoer minions unlike ).
Also, stay away from bot lane because you definetly will have a rough stsrt trying to learn how to farm in a 2v2 lane especially if both enemy adx and sup are pressuring you and your useless support(
and autofilled 
and 
mains LOL)
You'll get the most practice as ADC
I think the easiest would be a ranged champ against a melee champ in top.
Go into practice and just disable leveling and keep disabling and reenabling minions and just practice, obviously there is a bit more difficulty while while facing an opponent but getting just last hitting right at first helps.
Also, J4's passive does extra damage to any new enemy when autoing them. It's like an extra helping hand for getting it right.
top
Ad champs are easy.
his passive helps with last hitting a lot
has a solid amount of base ad
her mana costs are high early but she shoves waves really well
I'd honestly just suggest grabbing a pal and doing a 1 v 1. Good way to pick up on how to cs under poke pressure since no lane is going to just let you farm for free. If you'd like you can add me and I'd be more than happy to help out.
Top lane as nasus
Anyone who has an AA-reset or an empowered execute-type auto. 
, etc. Anyone who has the ability to do a quick second auto if you're off by a few will help immensely, and the more you play them the better you get at it. Every champ is going to feel different, and if you want waveclear versus precision that's a different monster entirely, but just practicing last hitting you want any of those listed. ADC is entirely different, where MF is the only one that really has an aa-reset that doesn't have another animation involved(mostly), but due to the ranged nature those aa-resets don't work as well on extremely low health minions, as melee's will hit before the projectile.
I'm a toplane/jungle main, and will tell you that Illaoi will have one of the easiest times farming once you learn to play around her tentacles, as q>w will clear an entire wave withing tentacle range, whereas w can for sure last hit cannons/minions you def dont want to miss, and also gives you the option to last hit then immediately reset auto someone that tried to harass you. Honestly it's gonna boil down to lane matchup regardless, and ap champs on average have a harder time early than ad, it's one of the reasons why ad's tend to shove against mages early game.
Mid to late game makes a great Wave Clear Unit. W on group of minions, and always aim for Cannon Minions as they lesson the CDR of W per tic a second.
Use 
. His aa animation is if not the worst, one of them. Generally used by proplayers.
Top Mid and ADC are all fairly easy to farm at tbh depending on your champion. Any AD champion has a pretty easy time last hitting. Most mages have waveclear that can just nuke the entire wave after you get some AP. You said you have a hard time vs assassins in mid, so I recommend you try my garen mid pocket pick. I take grasp of the undying and its impossible for assassins to trade with you or ever all in you. It's free farming for the entire laning phase and you just get stronger and more impossible to kill as the game goes on because of your passive giving you more armor and magic resistance. Zed is also another champion I recommend because his passive makes it really easy to last hit.
Practice cs'ing with cait at adc. Her range allows you to stay mostly out of enemy range to focus more on farming, and cait doesnt really use abilities to cs much so its good practice learning how to last hit with her auto attack because itll translate to many other champs. Thats my recommendation
there really isn't a role for it, its a bit different for every champion so you should practice it on the champion you want to learn.
Easiest - Cassiopeia -auto then e immediately after when minions are low
Once you master that I suggest Draven. If you master last hitting with draven and properly guiding and catching axes then you’ve mastered cs’ing
Midlane Mordekaiser.
Use Annie midlane. Use Q to last hit, autos before your last hit Q, and when you get better at it use autos alone as Annie to score last hits.
Heres a tip, keep an eye at how much damage enemy minions do to yours and which minions they are attacking.
I would suggest a ranged top laner. Top lane is the most isolated lane, and it's 1v1.
I started jg and sup and eventually branched out to Darius top. His Q and W make him a good champ to learn last hitting - generally easier than most ADCs. Hes a lot of fun to pick up if you havent played him before, and you can have perfect CS with just a little practice.
Honestly, practice tool against a single bot is the best way to learn how to farm. Don't attack the bot just cs in and out of being aggrod. This is what i did when i switched sup to top and it helped a lot altho I'm not that amazing at csing in bad matchups still
Dont listen to the people that say "go into practice tool" practice tool is so boring to learn in and a waste of time. to learn how to farm better just play nasus in any gamemode and just sit top and farm, wasting your time learning to farm wont get you to diamond, at best youd get up to gold but you will have to learn how to play the actual game
