I firstly want to say that most of the team styles that were relavent in reg M-A are still at least somewhat relavent in Reg M-B with exception being rain which has changed a ton since Reg M-A. I’m not going to talk about those team style in this answer because they are quite rare but if you want to read about them read this
Common team Archetypes
Staraptor teams
Despite tons of counters arising Staraptor teams are still one of the most common teams you will face as you journey the M-B ladder. There are also 2 kinds of Staraptor teams. Staraptor teams that are centered around mega Staraptor and have Pokemon like Whimsicott and Grimsnarl that can support mega staraptor as it sets up close combat boosts. The other kind of team are team that have staraptor as a second mega for taking on teams that lack a good mega Staraptor answer. Keep in mind that the latter kind of team sometimes run final gambit staraptor and if you don’t keep that in mind you can get seriously messed up by a final gambit sniping one of your teams important Pokemon. These teams typically include support Pokemon that can help keep staraptor alive and Pokemon that can remove ghost types that staraptor cant hit with Cc. The best counter to mega Staraptor is by far Ghouldengo as Ghouldengo isn’t scared of staraptors stabs and fits on most teams quite well. Froslass and Raichu Y are also good choices but they require you to mega them in order to deal with Staraptor.
Mega Raichu Y teams
The next most common mega is Mega Raichu Y. Mega Raichu Y teams are typically have multiple ways to protect Raichu because Raichu has about as much bulk as a toothpick so it really needs something to help keep it safe. Mega Raichu is typically not seem as a standalone mega on a team and is either pared with Staraptor or swampert. The best Teamates for Raichu are things that can help it avoid dammage (again because of its toothpick bulk) and Teamates that can deal with ground types. (Thats part of why it synergies with rain teams so well). Good counters for it include Sinitcha who Raichu can’t hurt and non mega Raichu.
This answer is far from finished and I’m just posting this now so I don’t delete this whole answer by accident.
