Any aerobatic aircraft will do to get started, so I won’t spend time on the basic aircraft here. Learn to fly whatever you have as well as you can, and learn the ‘craft’ of sport aerobatics on the way. There is a lot to learn. When you get to the stage of wanting/needing a more capable aircraft, then here are some options. I’ve stuck with the single-seat types as if you run an alternator and second seat you aren’t really serious....
If you have unlimited disposable income, you can skip this section!
Pitts S-1S,C,T,D: Very available at affordable prices. Lots of ‘corporate knowledge’ on how to fly one and their capabilities are well known. Flown hard they will require regular and ongoing maintenance. Four-aileron models needed for Advanced and symmetrical-section wings for Unlimited. Not truly competitive at Unlimited levels these days, but flown well on the right day can still be on the podium.
Yak-50: A classic aerobatic aircraft but getting hard to find at the right price.
‘Classic’ premium has overtaken their aerobatic price, I suspect. Can still perform well at Advanced level. Regular inspections needed on hard-flown examples.
Yak-55, 55M: Very affordable performance for the dollar, pity about the fuel and oil burn (as for Yak-50). Tight cockpit for tall pilots. Heavy ailerons and a big aircraft to push around the sky, and hangar. Good thrust to weight ratio, but drag makes them struggle at the higher levels.
SP-55: Rare but can be found at a good price on occasion. Should be competitive at Unlimited, but not in the league of the Sukhoi’s apparently.
Laser Variants: An excellent Intermediate/Advanced aircraft, though again will struggle at Unlimited. Price parallels quality of these homebuilt aircraft, and they hold their value well due to reputation and economical 4-cylinder engines.
DR-107: An excellent performing homebuilt capable of consistent Unlimited performances if built light and in the right hands. Economical to maintain and operate with good range and cruise speed to get to the next interstate competition. Can be bought for good prices in the US and imported. Buyer beware, of course. Drawback is time to build, if that is your desire.
Giles-200,202: Again, an excellent performing 4-cylinder design, unfortunately kits are no longer available new. Excellent performance for the dollar, but at the upper end of ‘affordability’ used, especially the two-seat model. Capable of Unlimited performance all day long. Excellent maintainability.
Edge-360,540S: The 360 is rare. The wire braced tail 540S turns up at a good price on occasion. The later cantilever tail model is hard to find and expensive to buy. They are very popular in the US but not used in competition elsewhere much (i.e. Europe). I think poor visibility is their weakest point compared to the European monoplanes.
Rebel, Staudacher, Hurricane et al: There were many ‘super-laser’ 6-cylinder machines produced during the 90’s in the US that can now be had for a good price. Most were conventional in construction (steel tube fuse, wire braced tail, wood/composite wings). Excellent value and Unlimited capable due to their high thrust to weight ratios, but may not have the roll-rate and handling qualities of the better respected machines.
CAP, MX, Sukhoi – go for it if you have the dosh.