A bike is a bike and as long as it rides it's good but if you want to do better then post ITT.

If you want recs for used bikes, post your craigslist and height. In a good market, you can get a decent older road/ casual bike for ~$200.
We can help you find something higher quality with more standard parts. It can be hard to tell the difference.
If you buy something, come back and post some pics of it.

If you look at a used bike, check the blades of the fork aren't bent back from the line of the head tube (crash), the frame for hairline cracks, the bearings all spin smoothly with no play, the wheels are straight and have evenly tensioned spokes, the cog teeth aren't badly worn (compare to each other and look for shark toothing), the cables aren't cracked, it brakes and shifts well and check the derailer cage doesn't move much when you wiggle it. Moderate surface rust is ok.
Most things aside from frame damage are just haggling points.
Take cash with you and haggle in person to get the best price.

>new road bike
A good one is around $700+.
Carbon fork. Aluminium frame. Not 'alloy' or 'hiten' fork.
New generation claris or sora is the entry level.
A modern hollowtech shimano or other crank is better than a cheap square taper.
Avoid triple cranks on new road bikes.
If disc brakes, look for spyres or hydros not tektro.
Most decent bikes are much the same, but a threaded bottom bracket and more tyre clearance might be points of difference.
example: Vitus Razor

>new hybrid
A good one is around $500+
try and avoid front suspension and get a rigid fork. Cheap suspension just adds weight and something to break
If you want comfy, run wide comfy tyres
quality rigid fork, chromoly steel or carbon, will also smooth the ride out
try and get an 8 or 9 speed bike, not 7 speed. At that point higher spec is more reliable and durable, not just lighter and better preformance.
If discs, try and get hydraulic discs
example: Marin Muirwoods