The first pair of striking gloves you purchase should be 12oz, 14oz, 16oz or 18oz boxing gloves that are comfortable on your hands and have padding to protect them during hard bag and mitt work
As a baseline, anyone who trains a striking sport needs to own a pair of standard boxing gloves. Typically these gloves are used for both boxing and traditional kickboxing, both of which avoid clinching and grappling techniques. It is really more about sizing.
These boxing gloves range from 12oz to 16oz, the most common sizes for bag and mitt work are 12oz, 14oz, and 16oz. The 8oz and 10oz gloves are generally reserved for professional boxing competitions, and lack enough padding to protect the hands during long training sessions or when throwing with power on the bag.
Anything bigger than a 16oz boxing glove is usually used for ultra-heavyweight training.
For general bag and mitt work you can use anything from 12oz to 16oz boxing gloves. If you have smaller hands a 12oz glove may be better. Remember that once your hands are wrapped, they will be much more snug in the gloves. Unless your hands are particularly small, most people can fill 16oz gloves when wrapped with a full 180in handwrap.
If you plan to spar, you generally want 16oz boxing gloves or the closest size you can fit in to for the sake of you and your partner. Sparring gloves generally have more padding on the knuckle area than gloves designed for competition. This will reduce some impact and protect your hands and your opponent’s face better than a stiffer competition gloves.
While some athletes spar with the same gloves they use for bag work, the high volume of impacts against the glove will compress the knuckle padding over time.
If you plan to compete in boxing professionally then you will need a pair of 8oz or 10oz competition gloves depending on the rules of your promotion.