With multi-classing its about two things
1. What you want your primary role to be
2. Whether or not you your primary and secondary stats correspond well.
The question then becomes less of "what do you want your primary class to be?" but "what do you want your secondary class to be".
If you want your secondary class to be a striker or defender then you are screwed. All of the best abilities from these classes come from class features and none of them can be replicated with reasonable success.
A multi-class rogue gets one sneak attack per encounter. A multi-class fighter gets one mark/encounter(and doesn't get combat challenge). A multi-class cleric/warlord gets one half the healing power of a cleric/warlord.
That means that there are two clear choices for secondary classes to multi-class into. The first is Cleric, the second is wizard.(Close Third is warlord, but that hurts a lot more than cleric does because the Warlord has better class features that tie into their powers and paragon paths more)
Once you know that then anything that multi-classes well into Cleric and Wizard(anything str/wis primary/secondary or int primary/secondary) is going to multi-class well.
Warlocks/Warlords multi-class well into wizards. Fighters, Rangers, Paladins, Wizards multiclass well into Clerics.
The other thing to look at is what paragon paths lend well to multi-classing(Wizards, Rangers, Warlocks are terrible at this for instance, while Warlords, Fighters, and Clerics are great)
Paragon Classes that Seem good for this are
Cleric:
Divine Oracle(Foresight! only one attack that requires a roll, reroll on attacks that target will, two very good encounter/utility powers)
Warpriest(Great for OA based Heavy Blade fighters: Warpriests Challenge = Whenever you hit with an at will melee power, you mark till the end of the encounter and get an OA against any enemy that shifts or attacks anyone other than you.)
This is not an immediate interrupt, its an OA, so you benefit from your wisdom bonus to attack. You benefit from heavy blade opportunity/Blade Opportunist and combat reflexes. Assuming you started with stats enough for those(14 dex, 16 wis, 16+str which should be possible), you get to make an at will attack against any one enemy you hit with an at will attack. Its an OA so you are looking at +9 or more to attack(wisdom + Heavy Blade Op+Blade opportunist+combat reflexes). And its an OA so it cancels movement if it hits via your fighter class ability.
This means you can make it impossible for nearly anything that provokes you to move ever(since you hit them once with the OA then mark them till the end of the encounter, and if they shift, you OA again!), or any one thing you hit with an at will to ever move if you so deem(because it can't move, or shift without provoking an OA[and the combat challenge, so realistically you could get two attacks and prevent two movements though one without the OA bonuses to attack], and with such a high bonus to OA you are very likely to hit)
Fighter:
Kensei(attack and damage, generic good melee multi-class for damage
Pit Fighter(AC and more damage, generic good melee multi-class for damage)
Iron Vanguard(A bit of extra hit points/defense for a striker as well as some pretty decent control/defender powers with all that pushing and shifting)
Paladin:
Astral Weapon: Enemies that dis-obey a mark take -2 to defenses. Nice in combination with a fighter which marks a bunch of guys.
Justicar: Specific Synergy with Inspiring Warlord level 23 encounter power.
Ranger:
NOTZING
Rogue:
Cat Burglar: Nice for defenders/front line leaders if your DM likes to use a bunch of movement powers. Good for high dex fighters. Possibly very good for high dex dwarven fighters(if such a thing exists)
Master Infiltrator: Nice for anyone with lots of AoE and wants to be invisible. Powers aren't that great though, and little synergy with anything else.
Warlock:
NOTZINK
Warlord:
Combat Veteran: For the front line Defender/striker who isn't hardy enough. High Con Dwarf + Combat Vet = Lots of hit points regained on a surge. Great str based attacks with stiff penalties on them. Encounter Utility power. Really amazing daily power.
Knight Commander: For anyone who can use heavy armor. Your allies gain +2 to attack when adjacent to you. That about says it all right there.
Wizard:
Spellstorm Mage: good for high int people who can be expected to be bloodied a lot. Class Abilities let you re-use powers.
Wizard of the Spiral Tower: Good for decent Int people who need the powers only. Since none of its attacks that have attack rolls use an implement. Otherwise fairly sub-par.
--------------------------
Yadda Yadda Yadda
Best options are Defenders/Strikers multi-classing into Controller/Leading/Defending classes.
Second is Warlord multi-classing into wizard(but not paragon multi or any paragon class)