What are the easiest visa options for retiring in Thailand, and how do they compare?

The easiest and quickest is TE. Also no yearly drama for the next 20 years..

There is a new Long Term Visa that just went into effect, with one option covering retirement, but it requires significant cash assets in order to qualify. US$1million in provable assets, and money coming in of at least bt80,000 a month. You also must have health insurance, but this gives you a five year visa that you can extend at least once, and with no 90 day reporting. There is also an investment visa that requires a background check, at least 10million baht in approved investments, and then just the regular bt1900 application fee. What is nice about this on as opposed to say the elite visa is you can actually make a good profit on your investments, something you can not do with an elite visa. The government does not want these investors to go broke, and is why the type of investments are limited.

Perhaps state how long you want to come for and what for...

I recommend getting Non-O Retirement.

There is no 'retirement visa'. Go for the Non Imm O or OA visa.

The absolute easiest will always be elite visa. You basically pay them 600,000 baht and they handle everything for you, as they should for that price.

The regular non-O is the standard method though.