Okay that is easy and hard to answer at the same time.
First the easy part:
Invest in models you like aesthetically. You will never (or almost never) regret buying models you simply like seeing.
For example me...I'll field at least 6 Stealth Suits in my ~2k list. Now Stealth Suits are far from being an actually good unit compared to the rest of the codex but they are one of my favorite Tau models.
Now to the hard part:
It really depends on what kind of playstyle you like and your local meta. Some people are fine with playing 6 Riptides because they are awesome models and it works out with their local meta without automatically becoming 'That Guy' but usually that would be over the top for casual/pick-up games.
On the other side some people like playing infantry Tau with lots and lots of Firewarrior and a shield wall, building a nice gunline and simply sitting there and shooting. Definitely not a playstyle i'd like and not one of the strongest Tau list out there either.
Normally I'd say you can't go wrong with Crisis, Riptides and Ghostkeels. However 8th edition is about to hit (about 2-3 months away most likely) and with the rumoured changes it's pretty much impossible to say how things will play out. Suddenly weapons that are just okay could become strong and weapons that are strong could become just okay. Same for units.
So I think the only really save unit will be Crisis Suits due their huge variety of weapons they can choose from. Riptides are very likely to stay strong but really....who knows what the future might bring. Same with Ghostkeels.
If the rumour of cover reducing accuracy instead of being a save-roll comes true and the confirmed(!) leak that armor modifiers are coming back I see it very possible for Burst Cannon Stealth Suits to become a really nice unit in 8th. Same with the Ghostkeel (which is already awesome).
Just imagine it. If the enemy wants to shoot your Stealth Suits he gets -3 to hit even out in the open (even Space Marines would hit on 6s then) while their Burst Cannons reduce armor by -2 (if we go by the SW:A rules). That's not half bad!
Of course that's a lot of speculating. Nobody can tell whether shroud and stealth keep working like that in 8th edition or that the burst cannon will have the same profile as in SW:A. But at this point we can't really do much more. 8th edition is too close to not think about it when planning your army but still too far away to know where things are going apart from the few leaks by GW.
Here is the link if you want to read the confirmed leaks yourself by the way: https://www.warhammer-community.com/2017/03/23/warhammer-40000-news-from-adepticon/
buy the models you like without paying attention to the rules since those can do a 180° with 8th edition. Only real save unit to purchase would be Crisis Suits. Riptides and Ghostkeels are likely to stay good. Stealth Suits have the potential to become a really good unit.