I did a lot of checking around and have found Europcar to work best for me. They've probably got more pickup & dropoff points than most, their rates seems comparable to others advertising cheap rates, and I've never had issues with the agents or the billing.

I don't believe there are issues driving a rental car into another EU country, but I haven't done so myself. In general, the pickiest country I've found for entrance & exit is Switzerland, but as you note you can rent on the French side anyway.

It's very easy to look up the rates on Europcar's site, and there are no hidden "gotcha" fees. It's all right out in the open. And one-way rentals are no issue at all.

If you're used to dirt-cheap US car rentals though, you won't find that sort of thing in France. No Hotwire $12.95/day specials. You're going to be looking at $30-$60/day, and don't forget that gas is quite a bit more expensive, so you may not want a very big car anyway (plus they're harder to park).