That is a matter of opinion. You should however look for one with an aluminum or magnesium deck. Avoid V series when possible... D and F series are both great engines....however the DURAFORCE is the most powerful.
A and C series are great head turners with a strong nostalgic appeal.
There are as many different opinions of which is the best model as there are lawnboy owners...so you would have to figure out what you want from from your mower...example...self propelled, or push....daily driver or restoration. Then when you get an Idea, we could point you in the direction of a good model for you.
Lee