Meganium-Honestly, there isn't much use for grass type pokemon in Johto, or even the Kanto side for that matter.
Jolteon-Really fast. Good special attack. Good team member.
Alakazam-Good speed and special attack. Crap defense. I'd go with a Slowbro.
Kingdra-Might be hard to obtain. It'd be good for dragon type gym and champion.
Arcanine-Unless you trade, you cannot obtain a Growlithe or Arcanine in Soul Silver. A good fire type alternative would be Houndoom. A Typhlosion would be good if you choose it for your starter.
Dragonite-Good all-around team member. Strong, fast, just great. Can't go wrong with Dragonite, except he is hard to train up.
Here's my suggestion for a Soul Silver team:
Typhlosion-Good fire type, as fire is limited in Johto.
Slowbro-Both water and psychic type. Good pokemon.
Dragonite-One of the best pokemon in the games. Can learn fire, electric, ice, pretty much anything.
Jolteon-Fast, good spec. attack. Good electric pokemon. If you want to get an electric type faster, grab a mareep.
Mamoswine-Despite its many weaknesses, it's tough to take down. Teach it ice shard (always hits first), so you can hit down Lance's pokemon.
Shuckle-Great defenses, making it a good defotter for healing up.
You can replace shuckle for a snorlax.
You don't really need to worry about your team in the metagame, but it's pretty fun to figure out. Have fun and good luck!