Hey I'm new to this metagame as I've mostly played smogon OU and I'm wondering how to get the hang of it and have some general questions about viability. Such as why iscress and P2 considered to be such good walls but in 6v6 there generally considered fairly outclassed especially with all of the previously uber powerhouses
Hey there
hobbert and welcome to Battle Spot!
There are many variables to consider when explaining the differences between Smogon OU and Battle Spot Singles, the biggest two are that this is obviously bring 6, pick 3 and that this has a very different banlist to that of OU.
The thing about 3v3 is that you can never be sure exactly what your opponent will bring into the battle from team preview, so it's best to bring stuff that has the best chance of covering several of the opponents threats at the same time, whereas in 6v6 you bring your whole team, and every team member is specifically designed to do certain stuff against certain mons. I guess stuff in 6v6 can be more specialised, whereas things in 3v3 are much more general.
Onto the banlist stuff, whereas OU has suspect tests and removes anything that is seen as broken or over-centralising, in Battle Spot we're stuck with the stuff that Nintendo has given us, so stuff like Mega Kangaskhan, Aegislash, Blaziken, Mega Gengar, Greninja, Mega Lucario, Mega Mawile and Mega Salamence run amock. The only things that are banned here are the title legendaries and event legendaries (the same as VGC and the Battle Maison), so no Mewtwo, Kyogre, Jirachi, Darkrai, Kyurem, Xerneas etc. Since we have to be able to deal with these 'uber' threats, but don't have access to defensive behemoths like Deoxys, Giratina, Groudon and Lugia, we have t o get a bit creative.
Enter Porygon2, Cresselia and Suicune! These three are some of the bulkiest Pokemon available to us, and are able to check a whole bunch of things. Cres has always been a staple defensive mon so I won't go into it too much, but it emerged very early as a great check to Mega Kangaskhan since it could avoid its non-contact option in Earthquake with Levitate and force it to hit it with Return or Crunch if they had it, which would incur Rocky Helmet damage. It also resisted Power-Up Punch so setting up on it wasn't always the best idea. Porygon2 sorta came out of nowhere as a super popular Pokemon amongst the Japanese community. It's got great mixed bulk with Eviolite, a great ability in Trace (allowing it to copy cool abilities like Parental Bond, Speed Boost, Rough Skin, Intimidate, Levitate etc.) and a cool movepool whihc includes all the tools it needs to annoy anything it wants. Foul Play lets it do plenty of damage to the physical powerhouses it's used to check, Ice Beam allows it to take care of the powerful Dragon (and Ground/Flying) types, Recover lets it stall out stuff, and it can either wear things down with Toxic or slow opponents down to help teammates gets past them with Thunder Wave. While the defensive set is good, the offensive set with Download is actually more popular according to the usage statistics. These sets try to come in on Pokemon with greater physical defense to grab a boost to their special attack and start breaking holes with Tri Attack / BoltBeam and/or Shadow Ball. The set is almost meta-defining, as any Pokemon that has equal (or really close to equal) base defenses will put remaining EV points towards their special defence so that P2 gets an attack boost instead! It's also damn slow and can set up Trick Room which can allow for teammates who appreciate it but don't necessarily need it up to succeed to destroy stuff for a turn or two (think Mega Mawile here). Suicune is the interesting one here since it's the newest mon on the block and has actually been jumping up the usage stats for the last couple of seasons (it's currently 9th after not even being there a few seasons ago). It has fantastic mixed bulk but prefers to go fully physical since it can easily get in a few Calm Minds if played well (and has Scald to patch up the physical side), it's pretty much the poster child for the 'try to cover everything with one mon' playstyle I was explaining earlier.
If you're wondering about usage stats and stuff, then check out the Pokemon Global Link's
Rating Battles section, it lists the top 12 most used mons (includes a search function for anything that isn't in the top 12) and shows the top 10 most used moves, items, natures and abilities, and also what that Pokemon defeats and is defeated by, as well as what else is on it's team! It's really handy and quite in-depth.
Wow that ended up being quite long!
tl;dr P2, Cres and Cune are all able to cover a bunch of threats at the same time.