There is never a black and white answer to such questions.  What Bob Crane and Dennis Swartz said make sense... and it depends on the market and your personal preferences and goals.

Class C usually brings in best cash, A normally appreciates with no cash flow.

It depends on your personal preferences - I own multi units myself, but other investors I work with prefer homes and condos

Many investment advisers will suggest that 4 plexes or larger are better due to the reduced maintenance cost per unit.

But in our town once you get past a duplex you multiply your problems with the city inspectors, so I stick with mostly single family and a few duplexes.

And Tenants will pay much more for a single or duplex than an apartment.

I'm with Nina Hollander.    Not aware of different classifications.  

We don't use those classifications here, but generally, whatever works for you.

I dont worry about class but look at income, expenses and how good of a deal i am getting on a sales price

It is all about the numbers and the investment goals. The only perfect formula is the one that works for each individual investor. 

parked and reading...

I have no idea what you are talking about.

investments are a numbers game.... if the numbers work and you have the capacity to manage properly, go for it....if not, you may be biting off more than you can chew...but it all starts with the numbers....

You need to work with a local professional it you are truly seriously wanting to buy something.

Sorry, my expertise is residental. Good luck

It depends.

Resale in the future & ROI now.

I prefer garden apartments with over 100 units. Big enough for a resident manager.

It depends on the particular investor and their situation.

It depends on your county and your local market. Four plexes are often a good bet but do the numbers in your area.

I think 4-plexes work very well. We have some 7 unit buildings and there is always someone moving in and out.

Many investors purchase C class properties for cash flow and appreciation.

There are so many variables here. Others have already stated this, but it comes down to your net. When you consider acquisition costs and expenses, additional income, management, maintenance, repairs, location, amenities, target tenant mix (to name a few) and your short and long-term goals . . . you'll know the answer for that particular project.

Devon mann - a couple of good answers. 

It depends on the market in your area

  Find an agent that specializes in multi-family.   See a CPA for advice on ROI.  Find out local ordinances.  Get a real estate attorney.

 Too many other factors to consider since every property is different and you need to consider location, condition, etc. Cash flow IS the primary concern.

Any income property is a wise investment. Its management that is needed

Devon mann Investments always come down to ROI.

What is your investment profile expectations - differs for each of us