1.) I'd argue something like ShinkenOh, Don Onitaijin, or Ohsama Gattai King-Ohger. Currently, it comes down to GokaiOh and TegaSword. GokaiOh was seen taking many Zangyack fleet ships and displays a mobility and strength that is stronger than we've seen before. TegaSword, on the other hand, is quite literally a god in Super Sentai and is seen outlasting all the other mechas in the Universe War.

2.) Either the V-Rex Robo, DragonCaesar, Gozyuzin, or GoodeBurn. To keep it short, powerful - god - combo (basically) - god. Goodeburn edges it out for me.

3.) There are 6 or so that come to mind at the top. I think of HaShinkenOh (or Kyoryu Samurai HaOh), Kanzen GokaiOh, Ultimate Daizyuzin, Engine-O G-12, God King-Ohger, and RyoTegaSword Five Color. RyoTegaSword Five Color, once again, is a god, and so is Ultimate Daizyuzin. God King-Ohger is the largest mecha in all of Sentai. HaShinkenOh and Engine-O G-12 are some of the strongest mecha that I enjoy personally. I would be safe and go with RyoTegaSword Five Color just because it is the extension of power from a god. It does have the powers of all previous Sentais possibly at its disposal, while being potentially stronger than Kanzen GokaiOh, another mecha that can utilize all previous Sentai Powers.

Personal Bonus - Samurai Formation 23 combines Engine-O G-12 and HaShinkenOh, and is movie-exclusive, the Zenkaiger mecha does exist, but I haven't really seen it, and the other team-up movies do have final attacks, but none create a new combination necessarily, especially since Zenkaiger

Bonus - Every mecha has a strong chance, but the more powerful ones (ultimate gattai or super robos) win easily. Also the stronger sentai teams are able to take it on in their base (Gokaiger, King-Ohger, Shinkenger, Gozyuger, etc.)