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There is no easy answer to this, as there are too many variables and the quality of the optics is very variable - there are a few that have outstanding optics, many very "ordinary" ones, and a few pretty awful ones.
The first and foremost rule is buy the most aperture you can afford. Then there is a practical limitation - the limitations imposed by transport/storage (will it fit in your car, and can you lift it safely if this is to be portable) or the size of your observatory (permanent).
This must be tempered by the questions of focal length, overall physical length (catadioptrics) and weight, and the mount required to support it adequately. Dobsonians are pretty straightforward, the only real questions are the focal ratio and how good is the mirror. But a refractor optic opens up a minefield of possibilities:
- cheap achromatic doublet, vs apochromatic ED doublet or fluorite, vs air-spaced or oil-spaced triplet, vs Petzval design ?
- quality ?
- focal ratio ?
Then the question of the mount opens up a whole new minefield:
- is this just for visual use, or astrophotography ?
- do you want any means of finding things (eg setting circles) analog or digital, or are you going to "rough it" with a star atlas in hand and a torch in the old-school way ?
- do you want GOTO functionality ?
- do you want it integrated with a smartphone or tablet (eg SkyFi connected to an iPad running Sky Safari) ?
- computer interface (laptop) ?
Then comes the question of whether to go for a catadioptric telescope, e.g. Schmidt-cassegrain (the Nexstar is an example), Maksutovs (which I use for lunar and planetary) or Maksutov-Newtonians (excellent for astrophotography).
Lastly, if price is not an issue there is always a Questar.
To answer these properly you must first decide what you want to use the telescope for.
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