PMD2's test usually gives me the Sassy nature if I'm honest with the questions. In Time/Darkness this means Meowth, who honestly wasn't exactly my first pick when I first got him, but I decided to stick with him and wasn't disappointed. Technician + Fury Swipes is super powerful in this game and you can tear down high-health enemies very fast with it. Unfortunately, a lot of bosses resist it (Omastar/Kabutops, Dusknoir/Sableye, Dialga, etc) and your coverage options aren't great until you start finding some good TMs. In Sky, Meowth is replaced with Riolu, who is a lot more fun to use in my opinion even if it kinda sucks to lose out on Fury Swipes. You get a lot more useful moves through level-up, and your base stats are much better than Meowth's. Another fun option that I used, but could only ever get by rigging the test, was Mudkip. Mudkip gets early access to Mud Slap, which lets you cheese your way through a lot of bosses by lowering their accuracy until they can barely hit you anymore. Later in the game you get access to Endeavor, which is honestly one of the most broken moves in the game, allowing you to 2HKO just about any Pokemon including bosses (I'm actually kind of glad they fixed this in PSMD so most bosses are immune to Endeavor, it was such a stupid, cheesy strategy).
I played through Gates once, and I don't remember what I used (I'm pretty sure my character was Snivy but I legitimately don't remember what my partner was. Pikachu maybe?). No real opinions on teams in that game, didn't like it enough to care, or to bother replaying it.
I only played through PSMD twice. The first time I got Froakie as my hero, and I believe I chose Torchic as my partner. The second time I got Treecko as my hero, and Cyndaquil as my partner. Both combinations were good, but I slightly preferred Treecko/Cyndaquil to Froakie/Torchic. Treecko's access to draining moves proved very useful throughout the entire game, and Cyndaquil's Smokescreen makes many boss fights easier, similar to Mudkip's Mud Slap from PMD2. Froakie was quite useful as well but really didn't get many great moves until later in the game, with your strongest attack being Water Pulse for most of the game unless you get lucky with TMs. Torchic really didn't seem to pull its weight very much, and I definitely preferred Cyndaquil with its Smokescreen support.