My opinion is none of the above! Landscape shooting would require a wider angle lense starting in the 20mm range.
For and XTi, I would use the 17mm-85mm that Canon makes or something close. The lower end Canon factory 18mm-55mm lense is not bad for landscape shooting, but has limitations.
Macro: Get yourself a dedicated Macro lense for flowers and bugs. Canon makes a great 105mm macro lense and Tamron also makes a 90mm and a 105mm. I've seen wonderful result coming from the 65mm macro if you can get close to the subject and 200mm macro for bugging so you can snap the shot a little further away.
Find a sales person that knows, or do a little research online. Cross reference pricing on eBAY before you buy second hand in a photography store. Also keep your eyes open in larger retail stores like Best Buy for OPEN BOX items. I've saved hundrens just by looking and learning.
Best of luck and looking forward to seeing what you end up with.

I purchased the Tamron 70-300mm Di last summer for $168 including shipping (and it comes with a 6 year warranty). For the price it's a gift. As long as you have enough light the quality is good (make sure you use a tripod at 300mm), some chromatic aberration is noticeable when the light drops but for $168 I would buy it all over again, best deal in the market. I took a lot of fun photos with this lens and even portraits are great.

I bought a 90mm macro Tamron lens a few years ago and it has been the best lens I have ever had for portraits and floral photography. It is a little slow and loud on autofocus, but has been a great lens optically. I would recommend Tamron if their quality is the same on their other lenses

I've seen Tamron really pushing it's 70-200mm 2.8. I really want a 70-200 for my 40D but am really struggling with going for the Canon L Series which seems to be the standard or paying nearly half price for a Tamron. What's the quality difference and does the Tamron have image stability? Any advice would be great!