IMCO
In my considered opinion
Example
IMCO, I believe that raising taxes would hurt our economy
IMCO, you're an idiot

Related Slang
IMO | In my opinion |
IMSO | In my sovereign opinion |
IMNSHO | In my not so humble opinion |
JMHO | Just my humble opinion |
m.02 | My two cents |
Hot take | A controversial opinion |
LBR | Let's be real |
TBH | To be honest |
You're most likely to encounter IMCO (in my considered opinion) on forums and social media, when someone is sharing their opinion about a topic. Using IMCO signals that the person considers their opinion to be informed and well thought-out.
IMCO is just one of many variations of IMO. Other variations include IMHO, IMBO, IMPO, and (perhaps the variant to which IMCO is closest) IMAO.