I think the two best as they age and grow into mature adults are 1) Banana's (and other banana combinations) and 2) BEL's (specifically super lessers)

The bananas because their yellow just gets richer and deeper with time and age (and doesn't fade out or diminish like other morphs. Plus you get the awesome black freckling which becomes more prevalent and visible with time and age - which I love (although not everyone loves the freckling).

The BEL's because they stay stark white and the bigger the pure white snake - the better in my eyes