define side arm? if your talking pistols then mysterious magnum, the police pistol if you have dead money, or maria. MM cause its pretty strong and ammo is uncommon but not rare and has illuminated sights, police pistol cause its strong and has 0 skill requirement and ammo is plentiful but its a hassle to get since you have to go through all of dead money for it and doesnt have illuminated sights, maria cause very cheap and plentiful ammo, illuminated sights large enough magazine and decent power for a 9mm, but its the weakest of the 3. Thats just my opinion based on my definition of good. Oh and cant forget the .45 auto pistol, silencer, strong, illuminated sights. its like the 10mm and the 9mm being married to each other
Depends where you are in the game, the 10mm pistol is great early and can stay useful with a silencer and the other mods and the ammo is everywhere. The 12.7mm pistol is good once you can get it, and can also be silenced. The unique pistols (That Gun, Maria, Lucky) are all very good in their own ways, but which is best depends on your character build. If you can get a hold of a Ranger Sequoia it's the hardest hitting pistol in the game, bu uses rare ammo.
If you define side arm as one handed weapon and include the energy weaons there are a slew of very powerful choices. The Plasma Defender using EC max charge rounds is probably the most devestating.
The Ranger Sequoia and A Light Shining in Darkness from Honests Hearts
Definitely A Light Shining in Darkness with the grunt perk