If you've already got Haeckel Sphere Set #1 then these four are the next ones I'd recommend:

• 6-color classic Haeckel Sphere (Stewart Coffin's original version)
• 10-color Haeckel Sphere (Stewart Coffin's new "Vexit" version)
• 6-color cubic Haeckel Sphere
• 5-color tetrahedral Haeckel Sphere (autumn leaves)