I prefer AGM batteries for their zero maintenance requirements and potentially longer life than flooded cell batteries. Lifeline is one of the leading manufacturers. If you would rather stay with flooded cell batteries, the Trojan and Interstate are 2 well known and popular brands. Be sure to get true deep cycle batteries, not the marine style.
My tongue-in-cheek apologies to my good friend, Ned, but I prefer the flooded cell Trojans. The maintenance required is a once per month unscrewing of the filler caps and maybe adding some distilled water to bring them up to their proper level. Two advantages to the AGM's are 1: they don't require any checking or addition of water, and 2: there's no 'liquid' to slosh around or spill; they can be mounted upside, down if you're so inclined (pun intended). There are advantages to flooded cells too. 1: much lower cost for comparable ampere hour batteries, and 2: arguably longer life; more deep-cycles before replacement is required. 3: less 'floor' space required for equivalent ampere hour capacity. Either one is a good choice. Some people will neglect to check their batteries on a regular basis, so for them, AGM's are a no-brainer. Otherwise, it's pretty much a toss-up.
AGM batteries are exactly the same size as flooded cell batteries for equivalent AH capacity. The Lifelines I have are the same size as the T-105s that they replaced. Also, if used with a good 3 stage charger and not over charged or depleted below 50% AGM batteries will give up to twice the charge/discharge cycles as flooded cell or the similar gel batteries (see here for graph), compensating for the higher cost. See here for a comparison of AGM to other battery types.
The biggest killer of batteries is neglect, and with AGMs that is almost impossible to do. Even at the higher cost, I believe AGMs are a better choice for most users.