Personally, I find that characters' Level 4 Limit Breaks are not necessarily their most useful Limit Breaks. For example, both Yuffie's and Red XIII's Level 4 Breaks are basically just like having a Slash-All or a Mega All Materia equipped and doing a Slash-All Attack. For Yuffie, I would be using her 'Doom of the Living' (second Level 3) Limit Break as it deals 15 continuous hits. With Red XIII, I'd also rather use one of his Level 3 Breaks - either Howling Moon to put him into a powerful frenzy, or Earth Rave for 5 continuous hits.
With Barret, as long as you have enough high AP Materia in his Missing Score Weapon to ensure you can hit for 9999, you're better off using Ungarmax (Angermax) as it deals 18 continuous hits rather than the 10 hits of Catastrophe. Thanks to a Facebook fan who pointed this out to me. Vincent also effectively uses the equivalent of either Slash-All or Flash when in Chaos mode - I prefer to use him in Level 3 Hellmasker mode where he will either do a few continuous hits or a status Attack.
With Cloud, Tifa and Cid I would recommend using the Level 4 Breaks. With Aeris, I think it's a toss-up between her Level 4 Great Gospel and her Level 2 Fury Brand. Fury Brand will fill both of Aeris' allies' Limit Breaks, which is wildcard in potentially a very powerful way.
Cait Sith only has a Level 1 and a Level 2 Limit Break. Between the 2, I would opt for Dice rather than Slots, as Slots is too risky considering that with some bad luck, it can lead to an instant Game Over.
Additional useful resource: STRATEGYWIKI.