Interesting question,as no one detector/coil combination can do it all for my detecting sites,i would say the DFX is my oldest and most productive detector as it is a nice detector to swing and also produced me the best finds and most gold,so that is a good barometer i guess that would make me choose it as my favourite detector.
I tend to carry a selection of detectors all ready set up for the site/s that i am going on that day,if its a roman site with lots of hob nails on from hob nail boots then a T2 with 5'' coil would be the weapon of choice,so that then would become my favourite detector for the day.If i was to choose one all round machine that i pick up and use most days,this is basically due to weight or lack of weight then that crown would go to the Deus and is currently uses on a carbon fibre rod that is light as a feather,but although is a very productive machine and ticks all my boxes its still not my all time favourite,that as yet is still the DFX as i find its a flexible all round machine and once mastered i use it as a switch on a go machine with the Hotshot coil and SEF8x6 coils.
We all have our favourite for different reasons,some like the fact that some detectors are easy to use,light and good at the 'specific' sites that they detect on,over the longer term its the quantity of decent finds and once again the DFX still being flexible can produce nice finds on roman,saxon and celtic sites.
So if i had to single just one all round favourite machine then i would vote the DFX,its getting on as it vwas one of the 1st batch of ten into the UK but still going strong and it just shows how good the build quality of Whites machines are.