As a general rule, carbocation stability depends on the number of substituents attached to central carbon. In isopropyl bromide, the carbocation formed is secondary carbocation, as Br group is the most likely to leave from isopropyl chain. The more stable carbocation would be one stablized by 3 carbon, or tertiary carbocation. The simplest of these would be t-butyl bromide which is carbon attached with 3 methyl groups
