although I voted Eevee for the sake of strategy in HG/SS I'd have to go with Leafeon, think about it. 10 years ago in the original Gold and Silver you would've been torn between starters.
Water starters are good but you'd bee weighed down with at least 2 HMs that only your starter could learn(If you didn't have a HM slave),
Fire starters are great but if you didn't choose it you'd have to wait until late in the game to get a fire type,
Grass is usually my pick because if you don't it's nearly impossible to find a grass type with good stats that isn't part bug/poison
since Leafeon and Glaceon were introduced my party for HeartGlod is probably gonna look like this- Cyndaquil,Wooper,Eevee(Leafeon),Mareep,Dratini and pidgey(basically a fly slave until I get Ho-oh)
P.S. In my opinion Flareon has horrible stats, mainly because of it's speed and Vaporeon was just (to me)that pokémon who was good but a different water type was always slightly better