>>33103721Colosseum Bonus disk can be used as many times as you like, if you get it you can basically get a bunch of Jirachis as long as you have multiple save files.
Celebi is infinitely harder to get, as it's truly event exclusive. Celebi events in America were in 2006, 2011, 2014, and 2016. That's 4 opportunities to get a Celebi. Jirachi has had very few American events yes, but there have been a ton of Japanese ones and the Wishmaker Jirachi is still pretty easy to get your hands on.
Manaphy is an absolute bitch to get (Ranger carts aren't like Wish Maker where you can get as many as you want, you get one per), but at least it's available in all 3 Ranger games, and then a 2016 event. Phione is just also having a ditto and like 10 minutes to breed, so it's technically harder.
I'd say it's a toss-up between Celebi/Manaphy and Phione. If the bonus disk didn't exist it'd be Jirachi, though.