For survivability, there are two formations that stand out: Cegorach's Revenge and Way of Heroes. For potency, Cegorach's Revenge and the Harlequin Masque detachment are ideal as the Solitaire can improve his chances of reaching combat thanks to Run and Charge from Turn 2 on. Based on these alone one can presume that the overall ideal choice is Cegorach's Revenge for the best of both worlds.
My choice, though, is to go with the Masque Detachment. I want to field the Solitaire so he can engage, not merely survive. A 24-30" threat radius from Turn 2 on gives you the freedom to pick your targets. Deep Strike allows you to arrive in a part of the board your opponent isn't prepared for you to occupy, and thanks to the threat radius it can be a conservative arrival point, to boot. You can direct him towards a critical target that can be vehicle or otherwise thanks to weapon flexibility. And you can also use him for a joint charge against a horde unit to cut wide swathes through it.
With the number of other threats you can throw out there--Shadowseers, Death Jesters and appropriately threatening Troupes--the Solitaire is an excellent "point and shoot" option while the rest of your army takes care of larger, more immediately urgent targets. I learned firsthand how devastating the Solitaire can be against Walkers (Defiler, in this particular case). Caress To Hits that glance on 6s is awfully nice, and the EW w/ 3++ means he can tarpit long enough to finish the Walker off in a couple rounds of combat.
Land Raiders, Dreadknights, Riptides... a nice long list of acceptable targets beyond the norm that the Solitaire can take care of while the rest of your army copes with the rest.
Rules of formations removed as a copyright precaution - Iris.