Level 100 Statistics (see level 5, 50, 100)
Tropius is basically a poor man's version of Exeggutor. It cannot use Swords Dance well either. If, perchance, its max HP was two higher, it could produce 101 HP Substitutes and then have a use, but unfortunately, it doesn't. 99 Base HP is very odd. Seriously, use Exeggutor for Sunny Day + Chlorophyll movesets. In Ubers, Tropius might be able to switch into Groudon's Earthquake and SolarBeam it to death or Swords Dance up, though it doesn't stand a chance of doing anything else and is still inferior to Jumpluff, who has access to Sleep Powder, Encore, and a myriad of other status moves.
Sunny Day makes Synthesis recover two-thirds of Tropius's HP as well as causing SolarBeam to only take one turn. Hidden Power Fire stops Steel-types like Skarmory from ruining Tropius's fun and is also boosted by sun. Add in the fact that Tropius's ability is Chlorophyll, and this makes for the best moveset it can run. However, Exeggutor still does this a lot better. Tropius's Special Attack is no where near Exeggutor's, so lower Special Defense Pokemon like Skarmory can sit and take its SolarBeams like nothing. Petaya Berry is an option because Tropius could use all the Special Attack boosts it can get, and with Synthesis recovering 66.7% in the Sun, there's not as much of a need for Leftovers. Hidden Power Ice does less to Steel- and Grass-types (as it doesn't get the weather boost), but Salamence is probably the safest switch-in to Tropius, so why not get it with Hidden Power Ice? It also hits other Flying-types harder than SolarBeam and Hidden Power Fire would.
Tropius will have trouble sweeping anything without the Speed boost from Chlorophyll because it is pretty damn slow, which is why Salac Berry is an option. For the last slot, Substitute could be good if Tropius was faster, and Synthesis isn't going to do too much good with Tropius's many weaknesses and the fact that there is no sun to boost it. There are definitely better Pokemon to Swords Dance with than Tropius.
You could try to raise Tropius's pathetic Special Attack with Growth, but honestly, it'll still be slow and the set up is too much effort with Tropius's lacking stats. You can use Leech Seed to help support Tropius, but the low Speed and mediocre defenses combined with high HP is the exact opposite of what you want in a Subseeder.
The Sunny Day versions will need Speed and Special Attack. The Swords Dancer will need Attack and HP / defenses instead of Speed because it'll be slow as hell anyway. Tropius's max HP is 402, so it gets shafted and is unable to produce 101 HP Substitutes.
Checks and Counters
Anything carrying Ice Beam. Even Suicune can survive a SolarBeam due to Tropius's poor Special Attack and then OHKO it with Ice Beam. The Pokemon Tropius should be able to counter with its top movesets, Water-types, can easily OHKO with a non-STAB move. Pathetic. Blissey, Snorlax, and other special walls, especially Regice, can easily defeat any Tropius moveset. Any semi-decent physical tank such as Weezing and Skarmory can and will laugh at Swords Dancing Tropius.