Dunno about relative prices in your area, but IMHO prime contenders
would be condensing gas and ground-loop heat-pump. (Oil gets pretty
demanding about regular service or makes a real mess, and max eff is
significantly lower than max for gas.) I'll wager that your gas unit is
at or below 60% eff on seasonal basis, mostly because of huge full-time
opening to draft-hood, so you can guess what 92% would do for you.
Regardless, you first want to find the installer you trust most to
steer you straight and do a good job.
And ... you might be amazed how much you'd save, with increased
comfort, by first finding and eliminating losses. Stuff like sealing
leaks, clock t-stat, vent damper, etc., etc.