Multiclassing increases the breadth of a character, but only adds indirectly to the depth/power.
This answer is written on the basis that "best" is roughly equivalent to "most powerful". Obviously, seeking a more "flavorful" or "rp-friendly" type of build requires a different evaluation.
For a casting class, the utility and power of higher level spells vastly outweighs almost any benefit gained from multiclassing.
While 5th edition does add multiclass progression on your "Spells per day" amounts, you still only have spells-known available as if you were a single-classed character in each class.
A Cleric 4/ Sorcerer 4 can only cast 2nd level spells in each class.
A Cleric 8 has access to 4th level spells. At 9th level, you add 5th level spells.
The power of higher level spells generally outweighs having a more variety with a few extra lower-level spells. Added to that, there is significant duplication between the spell lists.
For a martial class, the trade-off can be more balanced. While it remains true that many of a class' higher level abilities are significantly more powerful, the level of power is rarely comparable to that of a new level of spells. With a smaller degree of power trade-off, the increased versatility can make sacrificing the class capstone abilities seem more worthwhile, particularly so when a campaign might not be expected to last for 20 levels.
