This is an interesting topic, because I just had the chance to start over from scratch when moving from Olympus to Canon. I would not consider myself a professional, as I do not make my living from photography, and I do not sell digital images. I do sell some prints of my images, often up to 16x24 and sometimes larger. I mostly shoot landscape, macro and wildlife. I'm getting more into urban landscapes too, especially at night.
It works well for me having a high end body for my main use, and a small consumer body, mainly for my wife (she has some joint issues and cannot comfortably use a heavy camera/lens for very long at all), or as a second body so I don't have to swap lenses too often. I used to have an Oly E-3 and an E-510 (both 2x crop factors). I now have a 5D MKII and a Rebel T2i.
I had to get used to the concept of lenses behaving differently on either camera, and I think that makes a big difference when it comes to lens selection. Here are the lenses I chose for my new Canon system, and why I chose and use those lenses:
-Canon 24-105 f/4L IS: Came as part of a kit with the 5DII, and is a good bargain in the kit. It's a good lens for when I don't know what lens to use, and it's almost the same range on the 5DII as the 14-54mm on Olympus. I don't find the lens to be as useful on the T2i, it's not wide enough for many shots I do, and too short for wildlife and such. If it wasn't such a good deal as part of the kit, I would probably not have bought this separately.
-Canon 50mm f/1.8: Given to me by a friend who now has the 50mm f/1.4. Oddly enough, it's not a focal length I use much on the FF body (perhaps for candid shots at parties and dinners), but I really like it on the T2i for close range indoor sports. I am sometimes called upon to photograph martial arts, and the 50mm on the T2i works great. I used to use the Sigma 30mm f/1.4 on my E-3, but I get better results with the 50mm f/1.8 on the T2i.
-Canon 300mm f/4L IS: Bought new a few days after 5DII. I love this lens. Light enough to hand-hold all day (for me), quick focus, decent IS. I tend to hog this lens, so it stays on the 5DII most of the time. I've put it on the T2i a few times when I needed a lot of reach and it also worked really well. I haven't experimented much combining with the sigma 1.4xTC I recently bought (used, from a local seller). I would really
-Canon 70-300 f/4-5.6 IS: Bought new a few days after T2i. Mostly bought this for my wife, as the 300 f/4 is too big for her. This lens provides very acceptable image quality on the T2i in a package that is light enough for her to use. In the (rare) case when I think I could use something between 100mm and 300mm, I use this lens on the 5DII.
-Sigma 150mm f/2.8 Macro: Got a great deal for a used one on eBay. I absolutely have to have a macro lens for the type of photography I do, and this lens is very impressive. I knew I had to get a long macro to get the working distance I need on the 5DII, and I was really happy to find this deal. I also have a Sigma 1.4xTC I can use with it, but have not tried much as of yet. I can't imagine trying to shoot macro handheld, so the lack of IS isn't a big deal to me.
-Canon EF-S 18-55 f/3.5-5.6 IS: Kit lens with the T2i. I don't use it, but it makes for a very light combo for my wife on the T2i. Was worth it for the extra 80$ over the body only, but I never use it myself.
-Samyang 14mm f/2.8: I just got this, somewhat by curiosity because the reviews are great for an inexpensive lens, and partly because I think it could be great for some of my landscapes. Manual focus isn't really a problem for a set-up landscape shot on a tripod. It's only really useful on the 5DII though, there are better lenses in that range in for APS-C.
If I really had to take only a two lenses on a hike, I would probably take the 14mm and the 150mm macro with the 1.4xTC. That would cover me for landscapes, flower and insect macros and wildlife if I can get fairly close. But I know I would regret not having the 300mm when an owl or falcon finally shows up
I also really wish they would make a 500mm f/5.6 like in the old FD days. I'd love a lens like that, but the 500mm f/4 is too large for me.
Luc