As indicated by the four real estate agents, lending quesions are going to be best answered by lenders...
"It Depends"--This is actually a complicated question, which is the reason you do not have an answer yet (not to mention it is a national Holiday).
Some will say the first answer could be: Pay cash. Much easier than borrowing, but usually more painful for most homebuyers.
The second answer could be: Buy the home before you get married. None of the following will apply.
The next most obvious answer could be, go with a conventional loan, which requires the lender to take into account the "weakest link in the lending chain". In other words, once married, your fiance's questionable credit could result in the two of you being denied a loan, or could result in the lender charging you higher fees and/or interest rate to compensate them for the less-than-stellar credit. You will only know once you have applied. Applying does NOT require you to stay with your fiance on the loan--you can go back to just yourself.
Then, one looks to FHA. It can be a more expensive loan, due to relatively high mortgage insurance premiums (which one would expect, since not "pretty good credit" means that borrower has had issues paying their debts in the past; increasing the chances they will do so again with their mortgage payments.) FHA loans are partially guaranteed by the Government, with the idea of helping people who are not perfect candidates, get loans with little down and, possibly, some explanable credit issues, etc.
FHA tends to look at the "strongest link in the lending chain." In other words, get a co-signor who has higher credit, better income, more assets and they could help you getting a FHA loan. Arizona is a community property state, so the lender will be required to pull your fiance's credit and use his debts in calculating your ability to repay the loan (unlike conventional, wherein you can apply just on your own, thereby removing some of the negative of a spouse's bad credit, high debt).
The final "normal" loan category is a VA loan. Again, sponsored by the Government and designed to help those whom have served our country to buy homes under circumstances that would normally prevent them from doing so.
There are other types of loans out there--USDA, private money, other government-sponsored programs (City of Phoenix has $5 million to give away in the form of $15,000 grants to people buying within the City), etc., etc.
Coming full-circle: The agents are right. There are many factors to consider in your situation. Your best bet is to find a great lender (hint, hint...) and ask them to go through your exact situation and come up with the best loan program for your particular situation.