What do I think makes a good Pokémon team?
It really comes down to moves.
Sure, certain monsters have better stats than others, but you can become resonably good with any combination. I don't follow competitive battling rules and "teirs" and all that, I just want to have fun and be able to get through the game. And all teams will have weak points, so just pick a monster you care about.
I have nothing against legendaries. I use at least one (though no more than two) in each of my teams. But don't use a legendary just because it's a legendary, use it because you like that Pokémon - its typing, the way it looks, whatever.
That's the main thing. Any of the monsters can be viable (fully evolved is better, but not necessarily mandatory). It's about movesets.
And, really, this is what I make sure all my teams have:
1. Surf and Fly
Now, apparently I'm alone in wanting my main team to have all relevant HMs learned so that I'm never stuck. But I do understand since most of them are fairly useless as moves. Surf and Fly, however, are must-haves for your main team since A) They're the most useful in the field, and B) they're really powerful moves for their types. Surf, especially, is my go-to water move.
2. Earthquake and Dig
I put them together because they're both ground moves that a large number of Pokémon both Ground-Type and non-Ground-Type can learn, but they're on the list for very different reasons. Earthquake, because it's extremely powerful and reliable in a number of situations. Dig, because you'll never have to buy an Escape Rope again (and it's also a decent move in its own right).
3. A Powerful and Resonably Accurate Electric Attack
For Gyarados. Just for Gyarados.
4. A Powerful and Resonably Accurate Ice and/or Dragon Attack
For Dragon trainers. There will be several, and at least one will be in the Elite Four. The more attacks of this nature, the better and spread out across the team.
5. A Powerful and Resonably Accurate Dark and/or Ghost and/or Bug Attack
For those pesky Psychic types, who are stupid-powerful even after getting nerfed. If you're stuck in Gen I, I suggest attacking Psychics with powerful physical attacks (most of them have mediocre defense) and have a number of your Pokémon learn Bite (it was Normal in Gen I but Dark in II and beyond) in case you decide to play Stadium 2.
6. At Least One Paralysis and/or Sleep Attack
As in one that only inflicts the status effect. This is mostly for catching Pokémon, so Thunder Wave is usually my go-to. But various sleep effects are also handy since Ground-Types are immune to Electric.
For those hard fights, just in case.
8. Any Powerful and Resonably Accurate Attack of a Non-Represented Type
Just round out your team and cover your wekanesses. I tend to prefer Fire and Psychic myself, but there are a host of ways to choose. STAB (Same-Type Attack Bonus) is preferable, but not necessary if the stats are decent (In fact, it's ofen good to give a pokémon a move that's strong against a type it's personally weak against).
9. Stay Away From Stat-Affecting Attacks and One-Hit K.O. Moves
Yes, the NPCs spam them effectively, but unless you're really good it's best not to waste time that could be spent whittling down their health.
That's really all there is to it for me. With these tips, my teams tend to overcome all but the most pwoerful obstacles...and that's nothing a little level-grinding can't fix.