Here are some Pokemon with the ability Speed Boost, there aren't many at all.
So I've compared the defensive stats of all the Pokemon in this category, here they are:
- Ninjask - HP: 61 / Def: 45 / SDef: 50
- Yanmega - HP: 86 / Def: 86 / SDef: 56
- Blaziken - HP: 80 / Def: 70 / SDef: 70
- Sharpedo: HP: 70 / Def: 40 / SDef: 40
So as you can see, Yanmega and Blaziken are both bulkier than Ninjask and Sharpedo, so let's take these two out. As you can see, Yanmega has nice Defence, but bad Special Defence, Blaziken has good around stats.
- Yanmega is one option, 
- Blaziken is another.
So this means that both of them are the bulkiest Speed Boost users, however investing any bulk into these with EV's and Nature is not a great idea, as they are both pretty frail either way. Out of these, Yanmega is slightly bulkier in HP and Defence, whereas weak in Special Defence, Blaziken has slightly lower HP and Defence, however the Special Defence makes up for that lack of Defence. As I said earlier, investing bulk into these will do nothing. Hope I helped you to chose a Speed Boost Pokemon :]
