Top loaders seem to be the favorite here, but I'll throw in my support for the side opening panniers. I've used both the top loading and side loading panniers on two different bikes, and found that I actually prefer suitcase style side loading cases. I've found that they're easier for me to pack for a tour, because I can take them off the bike and open them just like a suitcase. Stuff stays organized in them, and if I have to stop and find something in them while I'm riding, it's easier for me to pop off the case, set in on the ground, and just open it up. All the contents are pretty visible right off the bat, so I don't have to start pulling stuff out of the cases to find something at the bottom.
I've been using a set of Chinese knockoff Pelican style cases now for years, and well over 100,000 miles. They were marketed under the Duratool name way back when I got them. Back then, I think I paid around $45.00 each for the large cases. I bought them from a company that is no longer in business, but there are other versions similar to these available. Even Harbor Freight sells a version of them under the Apache name; their large case is slightly smaller than the ones I have, but they are definitely a very well made case at a much lower price than a Pelican.
The cases started off on a V-Strom, and ended up on my S10 after I got rid of the OEM top loading panniers, which I never grew all that fond of. They're mated to a set of Happy Trail racks, with homebuilt quick releases. I believe the HT racks were in the $300 range, though it's been a decade since I got them and I don't recall exactly.
I went with the knockoffs because they were cheap and durable, but if I was setting up a rig from scratch in present times, I'd probably opt for the Pelican 1550's. They are about as bombproof as you'll find, and at about $220.00 each, they're a really attractive economical alternative to the high zoot aluminum panniers. At one time Caribou marketed an entire setup with racks and Pelican cases, but I don't know if they still do. Some guys hate that utilitarian Pelican look, but I actually prefer it.
This is what my current setup looks like. This was the day the bike rolled over 100,000 miles.