An alpha helix of a protein is most likely to be disrupted if a missense mutation introduces the following amino acid within the alpha-helical structure -
"Because of its small size, glycine often induces bends in a-helix".
"Proline, hydroxyproline, glycine, glutamate, aspartate, arginine, lysine, isoleucine, threonine, and serine are helix-destabilizing structure". 
"Glycine and proline are the helix-destabilizing amino acids".

Both glycine and aspartic acid can destabilize a-helix. However, glycine is much more destabilizing than aspartic acid (aspartate).

Amino acids destabilizing the alpha helix

Proline is never found in a-helix. The peptide bond of nitrogen of proline lacks a hydrogen atom to contribute hydrogen bond (proline is imino acid:- NH, not amino -NH2). When incorporated in this structure, it causes the chain to bend sharply (as at that level stabilizing hydrogen bond is not available). This disrupts regular helical conformation. Proline can only be stably accommodated within the first turn of an a-helix.
Glycine also tends to disrupt α-helix. Because of its high conformational flexibility and small size, it often induces bends in α-helices.
Amino acids with charged R-groups (i.e. charged amino acids):- They interfere with the formation of α-helix because of electrostatic (ionic) attraction between the charged groups. These amino acids are:-


Positively charged (basic):- Histidine, arginine, lysine
Negatively charged (acidic) :- Aspartate (aspartic acid), glutamate (glutamic acid)


Amino acid with bulky R group due to branch at β-carbon:- They destabilize α-helix because side chain of these amino acids projects out of the plane of the main a-helix chain. These amino acids are isoleucine and valine.
Amino acids whose side chain can form hydrogen bond:- These amino acids disrupt α-helix by acting as a competitor of the main hydrogen donor and acceptors. These amino acids are serine, asparagine, and threonine.
You should make it clear in your mind that destabilizing property of amino acid does not mean that they cannot occur in α-helix. They have a different propensity for α-helix.
For example, proline can never occur in α-helix as it cannot form a hydrogen bond, the major stabilizing force in α-helix. Glycine is also not commonly found as it is too small and induces bends in a-helix. Other amino acids destabilize α-helix when present in large numbers.

Propensity of α-helix (higher the number less the chance to be found in α-helix)

Alanine (0.0) > Leucine (0.21) = arginine (0.21)> Methionine (0.24) > Lysine (0.26) > glutamine (0.39) > glutamic acid (0.40) > isoleucine (0.41) > tryptophan (0.49) > serine (0.5) > tyrosine (0.53) > phenylalanine (0.54) > valine = histidine (0.61) > aspargine (0.65) > threonine (0.66) > cysteine (0.68) > aspartic acid (0.69) > glycine (1) > proline (3.16).
That means alanine is the most stabilizing unit.
Beside proline and glycine, any amino acid can occur in α-helix, however with a different propensity for α-helix and also with the different destabilizing property.
Alanine
Aspartic acid
Tyrosine
Glycine