I like Super Saiyan 3 Manectric and Megacham best competitively (at least till ORAS). If we're talkin about design im KINDA leaning towards mega herra. I dunno i really like it. It's hard to form an opinion on the ORAS megas till i use them, but tbh i like them all (though I like Swampy, Altaria, Lopunny, and Gallade especially).
Surf is always nice to put on your mons, Fly can be good depending on STAB options for your fliers (some mons only get Brave Bird at like 65, for example), and Strength can be nice for things with REALLY bad movepools, but otherwise yeah it's best to just use an HM Slave.
I find HMs such as Surf to be more of use than say FLY. It's why I mentioned fly out of like all the other HMs. I would take an offensive surf starmie over hydro any day simply because it's in-game. HM slaves are really helpful to navigate around, but most of them are not fitting on an ideal team. Well at least not my ideal in-game team