I had a lot of fun with the main teams I used through the whole game. (All I've really got left at this point is filling out the PokéDex and getting Trainer Card stars.) My team went through only a few phases, and during each phase, only one or two Pokémon were switched.
Current Team:
Typhlosion - Of course, my starter Cyndaquil.
Victreebel - I caught a Bellsprout as soon as possible because it covers a huge Water/Ground weakness my 2nd Gen. teams seem to have. It's also my Cut user. (Though I wish I didn't have to waste the move slot. :/ )
Steelix "Rocky" - That one guy in Violet town who trades you the Onix named Rocky... I always found him to be a huge help, and he still proves to be. He's my Strength and Rock Smash user. (Again, wish I didn't have to waste the slots.)
Lugia - Of course, the one caught from Whirl Islands. It just wouldn't be SoulSilver without it, now would it? xP
Nidoking - Ever since I caught it as a Nidoran Male from the route before the National Park, it's just been awesome. Nidoking has always been my favorite non-legendary, anyway, so it's great to actually have one on my team. (I did back in Red and Blue, but I haven't had the chance since then.) It's just got a lot of moves for Type coverage. (Earth Power, Ice Beam, Megahorn, Thunderbolt.)
Red Gyarados - Like Lugia, it wouldn't be SoulSilver without this guy on my team.
The ones left behind:
Sentret - (Taken off for Rocky?) I caught a Sentret soon after getting Poké Balls, as I'm sure most do. I don't think I ever trained it. xP
Beedrill - (Taken off for Red Gyarados.) On for a loooong time, since it was a Weedle. Until I got to Red Gyarados, it was a mighty part of my team. I really needed to use the HMs Gyarados could learn, so it had to go into the PC with much sorrow. :( It would definitely still be on my team right now if there was a 7th party slot. xD
Fearow - (Taken off for Lugia.) Fearow was caught as a Spearow back on Route 46 right after I got Poké Balls. It was a huge help to my team, and lasted as my Flyer until I got to Lugia. (I'd still have it as my Flyer if I had an extra party slot. I hate having both Aeroblast and Fly on Lugia.)