I have a booking with Royal Jordanian where they cancelled the first leg. It's from the EU, so EC261 applies. RJ are trying to re-book me on an alternative itinerary I'm not happy with. There is a much better alternative on a competitor, but they don't want to pay for it.
As I understand, I have the option of booking that better alternative myself and then claiming reimbursement from RJ. It's a risky option, but an option.
Under EC Reg. 261/04 there is no right to demand rebooking OAL just because you are not happy with the alternative suggested by RJ. If RJ has proposed a reasonable flight alternative, i.e. same day, despite inconvenient to you and you prefer another airline, RJ has a right to rebook you like this. Then you can only ask for a full refund and book on your own. So to take the stand that the alternative flight suggested by RJ is not considered reasonable and you have the right to purchase the alternative flight on your own and seek reimbursement of the costs from RJ, is extremely risky - and will fail if the court or national body do not agree with your assessment. I find it extremely unlikely that RJ will accept voluntarily reimburse if you buy another ticket on your own.