Help with exposure and histogram

[MadMax1412]

Hi guys,

I shoot with a Nikon D90 in JPG + RAW. When I post process (I'm still learning, so be gentle) I find I sometimes can't decide between a few versions of the same photo. I would like to know - what makes a good photo? Probably a very generic question, like "what's the best length for a piece of string?" and I'm sure the answer's going to be "Whatever makes you like it".

Here's an example of what I mean.

Original JPG photo from camera - histogram is around middle of window.
The JPG file created by RawThereapee when RAW file is opened and Auto-Levels is set - histogram is pushed to the left
The JPG file created if I manually adjust RT's automatic adjustments and darken it a bit - histogram is pushed more to the left.

Personally I like the slightly darker one (the third link), but I'm doubting myself and thinking it's probably the worst one because the histogram is more to the left then centred.

PS - I wanted to use the "Manage Attachment" options, but the dialogue box says I've used "5.19 MB of 488.3 KB Used" and I didn't want to delete any photos from there in case it meant they deleted from previous threads where they're used in, plus I'm not sure how to even delete them if I wanted to.

Thanks in advance.