Dodonpachi Resurrection if you like sci-fi setting. The other one for a more 'fantasy' setting. But seriously, if you are into shumps, just buy all the Cave products . All of them rock EDIT: might I add to that CAVE WORLD GAMES ARE ON SALE!? 50% off!!! You can actually get both Dodonpachi and Espgaluda 2 for the original price of 1.
I prefer Espgaluda 2 but since both games have excellent replay value despite being very short I would say buy both while the sale is still going on.
out of all the SHUMPs I own for iOS...my favorite still remains to be Space Invaders: Infinity Gene. Full of content and just never gets old. I hope they fix the iPad 2 bug soon because the wait has been killing me to play on the larger screen!
To answer the OP: All of them. Actually, I do prefer DDP, but only because I like the scoring style a bit more, and it does have a bit more content (because there are more playing styles in arcade mode, hidden bosses, a hidden 2nd loop, and a 4th unlockable character). In fact, DDP Res. was my favorite game of 2010. You really oughta get both (and Mushihimesama as well, of course), whoever you are. Well, maybe not if you're a 5-year-old girl, I suppose. But possibly still even then.
Both of them are awesome, instant classics!!! They never get old! You are going to play CAVE games in the future years to come if you like shmup games!
Is the bullet hell in ESP and DDP pattern-based or just a random onslaught? My favourites are the Touhou games but it seems most others, even when called bullet hell, don't follow the same formula.