For me, this has been quite a surprising Summer of Arcade... initially I thought it was an extremely weak line-up and the two games that I thought I'd least like (Lara Croft, Monday Night Combat) turned out being my two favourites this time around, and definitely up there in terms of overall arcade favourites.
 
Limbo - Initially seemed like the strongest release, the one I'd definitely be buying but as it stands that's my least favourite of the five... being only four hours long, offering pretty much no replay value which I suppose isn't too big a deal for an arcade game, but at 1200 points, it's a huge turn-off.  Doubt I'll go beyond the trial (unless I want the 1200 points rebate thing.)
 
Castlevania: Harmony of Despair - Seemed to me like the weakest release initially with the way the coop online worked and the fact it was only six levels, no XP and what not.  Turns out the game's a lot deeper than I thought, levels are actually quite long and seem like mini-SotN's and it offers quite a bit of replay value, hopefully it does well enough to merit some sort of DLC including new levels 'n' what not.
 
Hydro Thunder Hurricane - I haven't tried more than the trial yet, I will definitely buy this at some point if there's still a decent amount of people playing, the online on that game is a ton of fun.  For me, that kinda game sums up what the XBLA should be, short, fun and exciting games that work as well as any retail game online.
 
Monday Night Combat - So far my favourite Summer of Arcade '10 game, and probably my favourite overall arcade game.  Fantastic online fun, quite varied and in-depth, each class is a ton of fun to play and all have their own unique little quirks.  For me, the assault's been the most fun I've had wtih the game so far.  Only tiny complaint is the lack of maps, even though, as said on the Bombcast they all sorta follow the same mechanics so it doesn't matter too much but I have the feeling they'll eventually wear thin and become more of a chore to play.  However, Uber has promised some content in the future so hopefully there's at least one new Blitz map and a couple more Crossfire maps.  All in all, an essential purchase as far as I'm concerned.
 
Lara Croft and the Guardian of Light - Just picked this up earlier today, so far it's very strong - the coop mechanics are a lotta fun, feels fresh and works real well.  Glad there's finally a really fun coop-focussed game on XBLA, seems like there's a ton of replay value... the game sorta makes it that you have to go back and re-do levels to get new unlocks and what not.  Level design is cool, a lotta cool things happen in the game and you have to work together with your partner real quick in some situations.  Shame about no online coop at this stage, but I still seriously think it's worth the purchase for local coop.  I think this game'll probably be experienced best sitting side-by-side with your friend running through these levels trying to find the red skulls and such!
 
In summary, I think all of these are definitely worth the price point... apart from Limbo, with Lara Croft and MNC being the strongest two this time around.  So yeah, now that all five are out there for public consumption... what're your favourites thus far and why?

MNC 
 
Why?: The multiplayer, the voice work.... hell, I just like fucking shit up!

MNC so far but I havent tried Lara Croft yet but I might enjoy it a lot.

if i owned a X360 it'd be Limbo for me