Multiclassing into Wizard is going to be tricky. At low levels, it seems like a no-brainer for a Tactical Warlord to pick up either Thunderwave or Scorching Burst, but as you go up in levels, that attack is going to seem worse and worse since you don't have a free hand to hold an Orb/Staff/Wand, meaning you won't be getting the Implement bonus to attack rolls. A Warlock could use a wand, but there isn't any pact which is Int-focused, so Warlocks would probably be best off just adjusting their power selection to grab a multi-target attack or two.
In general, I think that the 'best' multiclass feats are going to be the ones that grant access to a class feature and don't depend on implements or stats. For instance, being able to use Hunter's Quarry once per encounter would be huge against Solo monsters, and it doesn't care what you are wielding.
In general, I think that the 'best' multiclass feats are going to be the ones that grant access to a class feature and don't depend on implements or stats. For instance, being able to use Hunter's Quarry once per encounter would be huge against Solo monsters, and it doesn't care what you are wielding.