I agree about the "depends on how it's mounted" comment, but only because it applies to all antennas. But that's where it ends.
The original question, highest gain for an antenna on an HT.
There's a 'depend' or two in that too. If the idea is convenience, there are no HT antennas with positive gain, they all have a negative gain. The more inconvenient the antenna is, the more gain it's 'likely' to have. Putting a 5/8 wave antenna on an HT means you will have some gain, not much, but some. Stick a several element vertical array on an HT means you will have more gain, and a LOT more inconvenience. And then, gian over what? What are you making those 'gain' comparison with, a standard 1/2 wave antenna, or a typical rubberduck? In a very 'rough' analysis, length equals gain. How much 'length' can you afford before you 'chunk' the whole idea??
- 'Doc
(I hate getting the antenna tangled up in chandeliers!)