Gravelor lvl 46
Who is going to be better at lvl.100 on Heart Gold.
Wheres your Starter??
Anyway, magneton will be best if you evolve it, in my opinion, with amazing movesets, and stats.
Cobat is close behind, and Tangrowth is ok, too.
You should replace your seaking with starmie, gyrados, or a transferred(posssible)Kyogre, give magneton the magnet, Gravelor the rough stone, and tangrowth the scope lens. Then youve got a pretty good team. (Magneton is the best)