Interesting question,as no single detector/coil combination does it all,i tend to use about 5-6 detector combinations,some are old machines that excel on certain sites and others are newish machines.But i will try explain below on what machines i use and why i use them on specific sites.
On roman.saxon and celtic sites that have had alot of activity on them,this inturn will be very trashy sites with alot of say roman hob nails from boots,so to get in between the trash i tend to use a couple of old Tesoro machine with smaller coils,these machines maybe old but the discrimination is super and the small coil allows you to find the silver/gold hammered coins.
Fresh water rivers,ponds or infact any water course had been used by the roman and they use to throw coins/artifact to please the water gods called votive offering,so i tend to use a very old Minelab Musketeer on a very long straight shaft and wear the control box around my neck,also i use the very small coil as well as this reduces he drag from the river flow.The reason i use this combination is they are very good at what they do and by wearing the control box high up it gives you alot extra depth when in a shallowish river hunting for these roman coins.Also this combination is a great Mudlarking detector for on the River Thames foreshores.
For general everyday detecting i use a Deus,T2 and DFX with a wide selection of coils and these 3 are used for about 80% of my detecting need.
If i suspect a scattered hoard is in the area,usually you start finding coins/artifacts in a tight radius and usually as man is going down deeper with more powerful tractors they are taking the tops of the hoard pots and scattering the coins,this then requires the heavy hitters to come out to play,i 1st use the Pulse with the stock 10'' coil on it then once i have taken as many coins wthat i can with that,then i progress with bigger coils till i get to the 18'' Coiltek coil.
Usually the Pulse will find me most of my deep targets,but if something i suspect is even deeper then i will bring out my Fisher TW-5 2 box to play and that will nail we hope the really deep targets.Although i must admit it rarely is used but like most things in the detecting world,its nice having the right tool for the job,no single golf club will do it all,its the same in the detecting world.
These are my main machines that i use and must admit i really enjoy using them and it works for me,of course if you are in say the Aussie outback or Alaska gold field then you would use different detectors and techniques.