Most people will consider the "best bang for your buck" to be Glock if price is a primary variable. They are usually low cost, have better performance than many other on roster guns, are easy to work on at home, etc. The new S&W M&P 2.0's are a close second depending upon the price. However, Glock Gen 3's don't have the CLI and mag disconnect. Next is probably one of the Beretta 92 variants. However, the plastic parts need to be removed. Of course, the unobtainable HK VP9 is also a great choice, but certainly more expensive. Glock is going to be lowest cost/highest performing across all variables. Other than that, I'd probably choose a VP9 as "best". Another consideration is to look at the Colt Python or Anaconda with 3" or 4" barrel. You're getting a very good revolver in a magnum caliber. Again, a "low price" is not the only consideration.
My take is different. Given the extreme limitations of the roster, and NOT considering a low price to be a high priority variable, the "best bang for your buck" is a high end 1911 from Nighthawk or Wilson Combat. You're going to get exactly what you want in 9mm, 38 Super, 45 ACP or 10mm. The gun will run, will be extremely accurate, and there are many options even within the roster. Nighthawk offers their "Express Gunsmithing Initiative" which takes an on-roster gun and converts it to an off roster model. Wilson Combat may do something similar. This sort of gun will outperform a production polymer framed handgun IF the shooter can take advantage of it.
To be clear, most people do not shoot well enough for there to be enough of a difference to justify a Nighthawk 1911 over a production gun like the Glock 19 or S&W M&P. They can certainly grow into whichever gun they choose. At that point, the differences of the 1911 vs production gun MAY be noticeable and worth the cost. But most people never get that far and their production gun is JUST FINE for whatever their purposes are.
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NOTE: Many people take exception to a high end 1911 recommendation. They hem and haw about the cost, parts, etc. That's fine for them and it's their choice. But, those same people go out and drop $5k on random handguns--milsurp to $1,000 Berettas--as if the TOTAL cost is nothing. It's like they are schizophrenic about their costs! They completely forget about the thousands of dollars they spent in the past year. They could have had one or two great guns for the cost of a pile of decent handguns.
Don't get me started on training and proficiency. After $10k in handguns, one would think they'd be able to shoot well. But, unfortunately, no.