'multi-purpose' lens for 600D

[Kitah]

Hi all,

I've recently been looking at lenses for my 600D. I won the 600D with a twin lens kit, but switching between the 18-55 and 55-250 in the field has been resulting in me missing a lot of wildlife shots (my main photography target) and I'm concerned about contaminating the lens and camera with dust and debris, even though I'm careful with everything.

So, I would like to get a reasonable lens if possible that has a reasonable range and is relatively versatile. I know the 'super zoom' lenses typically produce poorer quality photos, however as an amateur photographer, they would still produce better images than point-and-shoots or 'hybrid' type cameras wouldn't they? Having said this though, I'd still rather have good quality if I can, don't get me wrong. Just trying to put it into perspective if that makes sense.

What I photograph;
- Scenery, habitat shots, sunsets, sky, water scenes, water falls etc
- Wildlife; predominently reptiles and birds. Things like venomous snakes I have little experience with so will not approach. Also, many reptiles and birds as you guys would know often tend to disappear rather quickly if you try to approach so as much 'reach' would be good with the lens
- Pets; dogs, cats, birds, horses, everything both indoors and outdoors
- Macro; but I will be getting a dedicated lens for this (probably the 100mm Canon lens)

The lenses I was considering
- Canon EF-S 18-200mm f/3.5-5.6 IS
- Canon EF 24-105 f/4L IS USM (I know this will be the best lens, quality wise. Just concerned the 'reach' is inadequate)
- Tamron 18-270mm
- Sigma 18-250mm F3.5-6.3 DC OS HSM

I'm guessing but not certain that the tamron and sigma would produce poorer quality images? But from what I've been seeing in reviews, people seem to be fairly happy with the Tamron lens. I only recently 'found' the sigma so haven't looked in depth to its quality yet.

I know the L series lens I mentioned is the best quality glass by far out of them all, but I'm concerned that 105mm won't be adequate for what I need... Its just trying to work out whether the compromise on quality beats the reduced ability to zoom in on my 'targets'

Anyway, does anyone have any advice regarding these lenses, and any recommendations?

Edit: I should also specify that in the future I do plan on getting more lenses for more 'specialised' purposes

Thanks in advance,
Laura