What is the best GPU for Optiplex 755sff?

http://www.dell.com/downloads/global/products/optix/en/opti_755_techspecs.pdf

Well, for your question, you have an OEM system with an OEM PSU, & those are notorious for both their lack of modularity & lack of sustained power. Based on that, I would recommend a GTX 750Ti, a "low-power" GTX 950, or at best a newer GTX 1050. The common feature on all 3 of those GPUs is that they only draw power from the PCIe slot (75W max TDP), so you not only don't need PCIe power connectors (which I doubt your PSU has) but also can (generally) get by on a 300W PSU. You'll also have to make sure that the card fits in (see page 15 for dimension limits).

The other big issue is going to be your CPU. Even for some of those older games, I'm not sure that a dual-core CPU can cut it. However, it looks like your system can handle a quad-core, specifically the Core 2 Quad Q6700 (see page 10 of the specs). I would also strongly recommend upgrading to the maximum RAM; per page 12, you can handle 8GB of DDR2 RAM (2GB per slot).

The main thing that could be a problem is the PSU. From what I can tell, you have a 275W PSU (see page 28), but I can't tell if it's the standard or the high-efficiency. Unfortunately, I don't have enough knowledge to tell you if those stats will be sufficient for your needs or not. What you could also consider, though, is replacing the PSU. You'll have to make sure of the form factor, however -- a lot of OEMs use a proprietary size for their PSUs instead of the standard ATX, especially in small form factor builds like this one. ATX is 150mm long x 140mm deep x 86m tall. A common one for OEMs is the SFX (looks like a smaller ATX box, sometimes with the fax protuding from the top), & is 125mm long x 100mm deep x 63.5mm tall. What I would recommend is check the physical size of your PSU, then see if you can find one in the same physical size but that's at least 80+ Bronze rated, provides at least 400-450W, & is no further down than Tier 3 (if possible) on the tier list (http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html).