I played with Zevran Dialogue Fix, IRS:Alistair and Morrigan Restoration Patch which fixes dialogues in the game, and all three can be made to play well with each other with only slight modification. If you only run one of them though, and don't find Alistair or Morrigan particularly romanceable/likeable, I'd recommend ZDF which fixes a lot of other broken dialogues besides just Zevran's, though Ejoslin focused mostly on Zevran.
I'd also recommend Terra Ex's Silverite mines bug (if you like to play Awakenings) since I don't know if Bioware ever patched that horribly game breaking bug.
For 'must have's' that was my biggest 4 since they made the game much more playable and fun.
There are lots of other cool mods that are fun and add to the game, but I kept it to a minimum the game altering types. Ser Gilmore added companion mod plays nice with all those I mentioned, but check for incompatibilities if you want that one, as there are several, particularly any that add companions. I got all kinds of mods to add armor, make the UI more easily read/visible, CC additions like makeup, hair, skin complexions etc (Dracomie who did "Dragon Age Redisigned" has an excellent set of complexions I used in a lot of my mods--though I didn't use his entire mod, just the complexions), extra Dog Slot (which does play nice with Gilmore). And had many facial mods for NPC's I made in the toolset myself and installed them to my games.