I'd rate a straightforward flat barred road bike as the weakest of the choices available, you get any negatives associated with a road bike but lack the positives of multiple hand positions and aero position offered by drops. I'd have a good look at what sort of riding you plan, distances expected and whether you want some extra versatility in potential useage. Some of the options can really narrow down the search:-
type of brakes - if you want to be able to run properly bigger tyres then canti, v or disc brakes are needed - I'd rate discs as the way forward out of those
clearance and frame fittings - how big a tyre may you want to run, remember you can always bung on skinny road slicks as needed? Do you want to run mudguards, rack, do you compact or traditional geometry
frame material - carbon, steel, aluminium or ti - or a mix - or you don't care
For example, if you went for a fast hybrid, with disc brakes, rack and guard mounts, you could select a size that gives the ride position with say a 130mm stem. Then you could always switch to drops at a later date by using a shorter stem, you can get discs that work with drops and your brakes won't care what size tyres you've got. Get a frame with a decent sized head tube so that you can get an upright riding position. Maybe try out some alternates to flats or drops, butterflies, moustache, north road and flats with bar ends all have their fans.
This sort of bike can be roadified as much as you want, drop bars, inverted stem, skinny tyres, but a road bike(flat bar or drops) can't be hybridified.
Obviously the above contains a lot of personal bias
