In my experience the most fragrant are the pansies with the ruffled edges - they are sold under a variety of names but they all pretty much look alike (look for names like Rococo and Flamenco). The ones I found came in a mix of colors, grew tall stems (like 8 inches up), and were heavily ruffled. I haven't seen them available in the last couple of years so I don't know where you could get them. I saw them at Home Depot. I would shop around and give all the violas and pansies you see the sniff test.

do a web search for viola odorata. quite a few online nurseries list fragrant cultivars of these.
i have not seen them offered by places like the local box-store garden centers.

There is a series of "English" violas that are noticeably fragrant, that includes cultivars named "Etain" (yellow with lilac edge) and "Columbine" (white striped lilac). I find Columbine highly fragrant -- it's a perennial, though not tremendously vigorous. I notice that it is widely available this year and have purchased a few.

Here is a link that might be useful: English viola

Canyon Creek (online) has lots of viola odorata cultivars by mail; they're in northern Cal or Oregon, I think. Thompson and Morgan has a selection of fragrant blue violas.