depends on what tier you want to play in.
However, an all Starter team would share a lot of weaknesses, as at its most diverse, it would have two fire types, two grass types, and type water types... Probably the best all-starter team would have to be all the kanto starters and all the Sinnoh starters. best type diversity (from secondary types, you get access to Steel, Fighting, Flying, Poison, and Ground). The Johto starters all suck... you could use Swampert and Sceptile instead of Torterra and Empoleon but IMO the latters are much better pokemon in terms of what they can do.
an all uber team would only be permitted in, well, ubers. so you'd only be able to use that team against other people with their ubers. Remember that Articuno, Zapdos, Moltres, Suicune, Entei, Raikou, Regice, Regirock, Registeel, Azelf, Uxie, Mesprit, and Phione are legendary but not uber. you can still use them in ubers if you want but it would be really hard. Heatran is also in this category but is much stronger and competitively sound and has better stats, and can be used with a lot of success in both standard and ubers. Its more useful in ubers there because its weaknesses are far less common (Fighting and Ground are almost non-existant in ubers save for Groudon and the respective version of Arceus and the odd Aura Sphere here or there, Water is still around but you most likely won't be seeing water attacks unless you are confronted with a Manaphy, Kyogre, or Palkia, in which case simply switch or pair up with Groudon to nerf the water attacks)
because there are only like 30ish pokemon in ubers, uber teams are less about type diversity and more about making sure you have the pokemon and the moves to counter all the other ubers (which is easy because there are so few of them) so whatever shows up, you will have something to counter it. therefore its not exactly the end of the world if you have two flying or psychic or dragon types on your uber team (which is almost expected as those types are so common in ubers and there are like 5ish pokemon that aren't at least one of those types)
IMO, always have one of the triad from RSE on your team (Groudon/Kyogre/Rayquaza)... as they are great team players and can set up a very effective strategy for the rest of your team, or in the case of Rayquaza, foil your opponent's weather. having all 3 on your team wouldn't be a bad idea either, as long as the other 3 pokemon could benefit from the weather effects. that would require a lot of prediction and switching to set up the weather for the proper pokemon to use so on second thought maybe thats bad.
Ho-oh should always be considered as it has a signature attack that is now physical (ALWAYS emphasize attack over sp. attack in ubers because the majority of ubers are special based and have pathetic defense stats in comparison to their special defense) and has a 50% burn rate, AND has a blisteringly high sp.defense (again, emphasize physical in attack, and special in defense)
Dialga makes a perfect tank and has the offensive stats to match... Palkia should only be used IMO if you are using Kyogre... although you could also teach it flamethrower and pair it with Groudon and give your opponent a big surprise...
Lugia is another great tank and being a psychic type it is a decent counter to pokemon such as Mewtwo and Deoxys. speaking of Deoxys, its also a great idea to fill in your 6th slot, and use the version that your team needs the most (a tank if you are missing one, sweeper if needed, etc.)
you get the idea. oh, and you really can't use a mixed team. you'd be mixing mere OUs and ubers and if you faught in ubers with that half your team would get wiped pretty quick and if you fought in standard you'd be basically cheating.
