This is my personal build for Mei. You can choose to build it or not. It is entirely at your discretion.
Personal Roles: Pseudo-carry/Nuker/Lane Control -She has outstanding lane control, especially with her devasting nuke. It can even damage buildsing. The reason why I stated pseudo-carry is because you may have a main carry being an agility hero or something similar to Raikage.
At the beginning - Level 1-11
Beginning Item Build:
-Shin Guards
-2 pouches
-2 Mana Pills for fire spam
-Courier
-3 Meats
After 3-4 minutes in your solo lane, you should have bottle and a full courier by now. Keep your courier at the lane until you fully use the bottle.
Sub-Beginning: This is around 11 minutes into the game. 11-16. This is the period where your farm matters the most.
-Shins - You don't need Remi in a pub game. Only in an inhouse where it matters since it provides a health bonus/regen.
-2 Pouches
-Bottle - Keep it full ALWAYS if you want to live. It has helped me SO many times.
-Int Treads - Why? Intelligence is required for Mei to be at her maximum potential. It fills its role.
-Amp Watch
Mid Game: 17-30 - Core
-Shins - Sell later in mid game. Again, if its inhouse, get Remi.
-Amplified Silver Watch - Faster HP/Mana Regeneration.
-Bottle - HP/MP regeneration
-Int Treads
-Karites Touch - it is a triggered spell. It provides a bonus for your ultimate. It also provides a significant damage to their int supports/carries. Should be maxed 5-6 minutes after you get it.
-Asassin's Cloak - Another triggered spell. Hit them to let the crit go and then use your combos for it to stack! Crit, Globs, Wind, Fire, Karites!
Late Game: 30 and beyond. You should be on a streak and farmed by then. Your fire does significant damage to towers for lasthit. I'll move onto the luxury build.
-INT Treads
-Maxed Karites Touch - Ult bonus
-Assassin's Cloak - Ult bonus
-Chakra Armor - it blocks single-target spells. It is useful against someone who has a disable. Also, it gives +10 int to your ult.
-Sage Scroll - triggered spell. It is a hex and it provides a bonus for you and your ult.
-Lunar Blade, it is a brilliant orb. It slows and provides a good intelligence bonus.
Optional:
Ikazuchi - Greater attack speed. You will DPS/kill the person faster at the risk of being unable to catch a swift hero. It is also a trigger spell which increases your attack again.
Skill Build:
1. Level 1 Fire (e) - Provides good DPS at level 1. I think its OP.
2. Stats - Why? You need intelligence for that ult of yours! It also helps a bit with lasthit.
3. Level 2 Fire (e)
4. Lava Globs (r) - Keep it level 1 until you reach level 16. Why? Your mana is lacking early game. You need mana to sustain control of the lane. Also, it provides sufficient damage even with your ult if you have said items above.
5. Level 3 Fire (e)
6. Ultimate - Use globs at this point after your fire on the enemy. You deal more damage if your ultimate is triggered. Use Globs after your E.
7. Level 4 Fire (e)
8. Stats
9. Stats
10. Stats
11. Ultimate
12. Stats
13. Stats
14. Level 1 Wind - Use it for reduced mana cost. Your regen is now sufficient enough to sustain.
15. Level 2 Wind
16. Ultimate
17. Level 2 Globs
18. Level 3 Globs
19. Level 4 Globs - You will do a lot of damage if you triggered the other spells first. This skill is your main DPS.
20. Level 3 Wind
21. Level 4 Wind
22-25. Stats
Order of Operation- This is significant because it gives you a large amount of attack in seconds.
1. Cloak --> Wind --> Fire --> Karites --> (Sage late game) -->Globs
2. Rinse and repeat. You will have around 2k mana and HP with 300 attack.