Man, I *live* for pressure wave clitoral stimulation (no, not sucking on the clit, it's different); I tried one once and I've been a fan ever since, literally trying to get my hands on every single company's take on it out there. Vibrators work nicely too, but they can't beat pressure wave stimulation for me. Everyone has their preferences, though, so you may want to get several.
These were already mentioned, but I want to throw in my two cents as well. In no particular order:
Satisfyer Pro 2 is relatively inexpensive. It also feels relatively inexpensive compared to the others. It tap-taps the clit kinda average, doesn't hurt, and has a variety of speeds (x11, if I'm not mistaken), but no patterns. It's a good 'tester', if you wanna try clit stim of this sort.
LELO's Sona basically takes the Satisfyer Pro 2 to the next level and hammers away at the clit. Hard, strong, and fast. Unlike the Satisfyer, which I usually get up to speed no. 8-11 to orgasm, I can sit at speeds 1-4 on Sona & come just as smoothly. Several notable reviewers have mentioned that the toy tends to slow down/stop when pressed hard against the clit, but I have not experienced this issue. Other reviewers have claimed that it's so strong that it injured their clit (to the point of blood -- make sure you use water-based lube to avoid this). I have not injured my clit yet, but it has become strangely itchy after orgasm with Sona a few times. This itchyness goes away after like 5 minutes. Get this one if your GF has a clit made of steel.
Womanizer Premium is like a lovechild between Sona and Satisfyer. It taps away stronger and harder than Satisfyer, but not as strong as Sona. The weakest settings feel more gentle than Satisfyer's. The strongest ones are about as strong as Sona's 5-6. It's got the most speeds out of the 3. It's also got an autopilot function if you want the AI to take control of the speeds. In case you're worried about noise, this model is the most quiet.
All of these are waterproof, rechargeable, and the bits that touch skin is silicone (that means water-based lube only). Super easy to clean too.