Like most things, it depends.
If you want to shoot from a bipod and rear bag, in a more 'limited' class, then FTR limits you to either .308 Win or .223 Rem.
Open class is pretty much anything under .35 caliber (or within limits of the facility you are at), shot from a front rest and rear bag.
If you go strictly by what's winning most big state or national level LR tournaments currently, you see predominantly .308 Win for FTR, typically a 200 gn projectile of some sort, and for F-Open you mostly see straight .284 Winchesters (or something close to it) running 180-184gn pills.
That's not to say that the right person, on the right day can't clean house with a .223 running 80s, or a .308 running 155s, or a straight 6 BR running 105VLDs, or a .300WSM running 215s or 230s. On the one end, you really have to have your wind foo dialed in tight - tighter than everyone else - and on the other end, you have to be able to manage that level of recoil and torque over an entire day or weekend with no lapse in gun-handling. Bringing things back to 'mid range' (300-600yds) opens up the performance envelope a *lot*, as you're less dependent on ballistic horsepower and wind bucking, and more on raw accuracy - and there are a lot of combinations that will shoot just fine on a calm to moderate day at 600yds and less.
Dang near anything can be 'competitive' on any given day at a local club match. A lot of times, familiarity with local conditions may trump the amount spent on gun or gear. It's when you get a number of really good shooters all together, who all have the ability to read the conditions and the experience level to apply that knowledge... that the $$$ rigs start to pull ahead. Not always, but more often than not.
You can get started, and see if you like it enough to commit more fully, possibly using something you already have. A lot of people have gotten started using a Savage 12 FTR or F-class, and just rebarreled after their first year once they had a better idea what they were looking for. Some people stick with that, others may sell it to a new shooter or keep it as a back-up / loaner gun and have a full-custom gun built to scratch that itch.