Penang or Kuala lumpur

1) There is one problem with both. Malaysia quickly became the “trash can” for developed nations since a Chinese ban on waste imports from the beginning of this year. The plastic recycling industry will earn Malaysia $841.95 million this year.
Plastic that is unsuitable for recycling is burnt, which releases toxic chemicals into the atmosphere. Or it ends up in a landfill, potentially contaminating soil and sources of water.
The stench of burning plastic and fumes from nearly a dozen recycling factories wafts through the neighbourhood. The situation is getting worse, especially with more and more illegal plastic recycling factories.
If the wind blows in the direction from the port Pulau Indah, from Kuala Langat and Selangor, you will smell toxic fumes. Penang has this problem too.
You can read about this in "Plastic waste industry worth RM30b" (in themalaysianreserve) and there is a good article in reuters "Swamped with plastic. Malaysia struggles as global waste piles up".

2) Penang is pron to floods since it is almost at the sea level and storm waste water doesn't have enough time to escape in the ocean. Kuala Lumpur is much higher above the sea level.
3) Mud slides is an issue there. Some constructions were ruined due to mud slides in Penang.
4) In Penang, there are several developments on reclaimed land, which is pron to flooding.
5) Also, research "tsunami in Penang". It is an issue there too.